Thought I'd say a few words on the qualities of the JMP-2 and Ztron-CLT3 V1.0  timers.

First off, you can't really go wrong with either timer. Functionally they are very similar and costwise the same $30.

Both have the functionality of being able to program in a dwell time, the time between arming the system, and when the power is going to be applied. Think of this as the time between raising your arm to signal the judges, and picking up the handle in the center of the circle. Since you are trying to conserve power, it is useful not to be wasting electrons during this time period. The JMP-2 allows you to program in a non-zero throttle setting for this time, but I have always kept tht throttle at 0.

Also both have the functionality of setting a throttle setting at the beginning of the flight time, and the throttle at the end of the flight. This is useful if you fly with a speed controller that does not have a governor capability. In these cases, as the battery voltage drops during the flight, a constant throttle would actually give less power and rpm at the end of the flight. So you set the end throttle somewhat higher than the beginning throttle. The Timers linearly ramp up the throttle from the beginning value to the end value, more or less cancelling out the drop in the battery voltage.

If you have an ESC with governor mode,you would typically set the beginning and end throttle values the same, and let the ESC regulate the motor to give a constant rpm. The JMP-2 gives you 64 throttle positions between off on full on, the Ztron 50---nominally the same. However because of the way the ESC handles the governor mode, sometimes I wish for a little finer granularity. For example, when I used the JMP2 on my Electric SuperCLown, with the stock Brodak Motor (kV=1500), I had to use what was called "low governor mode" on my Castle Creations Phoenix 35 ESC. In order to get the rpm in the right range, I had a delicate adjusting dance with prop pitch and timer throttle. In this case I was getting ~700rpm jumps between each throttle click. This is primarily due to the ESC governor being really setup for RC helicopters, where the gearing is such that rotor rpm is relatively low, and besides they have variable pitch rotors to adjust. The Ztron would have been slightly worse in this case, since its throttle click is slightly larger (2% a click vs the JMP-2 1.6%). One feature I would like in either case would be a finer adjustment (lets say ~100 steps at a minimum), or possibly, a way to set a range that the 64 or 50 steps would be adjusted between. So that's a wish!  added 26-10-09  I struck out the preceding sentences because the problem was basically related to the particular motor on the SuperClown. The governor keeps track of rpm by counting the magnets that rotate by. On the SuperClown motor, there were 10 magnets. On most commonly used motors nowadays (Axi, Scorpion..etc) there are 14 magnets. This allows you to use the high mode which has more resolution per "tick". So this issue is largely resolved.

Both Speed controllers can give warning blips at the end of the flight to alert you the power is about to drop off. The Ztron allows you to turn off this functionality. I like this functionality because the darn blips sometimes almost cause you to have to land, and if they occurred during the 4-leaf clover for example, might cause a crash. I actually fly with a "Talking timer" which counts down during the flight, so I know when the power is about the turn off.

Both times allow you to set a flight time in 10 second increments. The JMP-2 has a few more options in this regard. It divides the Normal flight time into two segments. During the first segment, the throttle stays constant, and during the second segment, it ramps up. The Ztron begins to ramp up immediately (if you have set the final throttle setting higher than the start of course). Since I use governor mode, I don't really notice the difference as I keep the same throttle setting for start and finish. The Ztron has a switch which sets the flight time whenever you arm the timer. So if you want to change the total flight time, you simply need to change the switch setting. For the JMP2 timer, you need to go into programming mode and have a stop watch available to change the time. This is a bit less convenient than the Ztron.

The biggest difference between the 2 timers is the way you program the times and throttle settings. As mentioned, the Ztron has a 8 position dip switch. I believe the Ztron is easier to setup, or lets say, less fussy than the JMP2. In both cases, you want the instructions nearby, because it is easy to get confused, which makes changes at the field a bit tricky. In either case I recommend you make up a servo indicator. Briefly the way this works is that you hook an RC Servo to the Timer connector that you usually use for the ESC connection. Both the JMP2 and Ztron have a retract capability, which I doubt most of us will use. However this connector can be used to connect a 4.8V RC nicad receiver battery. When you do this, the servo arm will be used as a pointer to indicate what the ESC would normally see and do. I mounted my servo on a piece of plywood, and glued a T-pin onto the servo arm to act like a pointer. When I arm the timer, you can follow the flight, how long and how munch throttle. By making a pencil mark on the plywood, you can see the relative throttle settings as you change parameters. Believe me, this is a lot easier than dealing with a spinning prop, especially when you have zero'd in on where you want to be, and are making small adjustments. Also it doesn't burn down your lipo.
In order to program the JMP-2, you need to pulse a single push button switch to adjust the individual settings. So by pulsing the push button 2 times and holding, you set up the capability to set the total run time. When you release, the number of seconds between the release and the next set will give the flight time (actually flight time 10 X the # of seconds). So for a flight time of 5 minutes , the elasped time would be 30 seconds. Ti set the final flight speed, you do the same, but after the release, you repush the button before a second has elapsed, the throttle (servo arm) will begin to move. It will step up from the last setting. If you go to far, briefly releasing the button will cause the direction to change. This is where you need to mark the desired position on the plywood servo platform I mentioned above. I really can't imagine doing this with a real prop. There are too many things going on.
The Ztron is less brain intensive----you hold the push button on when attaching the battery, then set a setting on the dip switch, push the button again, then reset the dipswitch buttons for the desired setting, then push the push button to record the setting.
I like to run the flight through on my servo setup to verify flight times and make sure the  throttle settings are nominal.

When I use governor mode, when I think things are set nominally, I will put a small prop on the motor, and hook up the system. The reason for the small prop is that sometimes the larger "flying" prop exerts a pretty high load on the system with the plane held down. A brief run is ok, but if you want to test the flight out, the amps and power you draw with the plane at rest is pretty high, and you could overstress the ESC and the batteries (especially their capacity). With a smaller prop (1" or 2" smaller diameter), the ESC will be able to set the run rpm without putting a lot of stress on the system. If the rpm isn't where I want to run it, I take the timer off and put it back on the servo system and adjust it a step or two up or down, then put back on the plane. That's how I start the setup. Of course when I fly I may find the lap time too fast or slow, so it may be back to the drawing board. I think this is one of the negatives of both timers, I find it a little scary to try and set things at the field, without slowly taking my time and thinking (and rethinking) each step. It is important to take notes and keep track of the Ztron dip switch settings, or in the case of the JMP2, the position of the servo pointer.

Sounds fussy---yes it is, but once you have it, you have it!

One other negative for both times is the way you plug in the ESC connector. This is a RC servo-like plug (three pins), but the timers don't have a polarized housing (or any housing). You need to be alert and careful how you plug the ESC plug onto the timer. It is easy to get the switch upside down, or one pin offset. This last thing happened to me when I was a little too hurried---it toasted my JMP2 timer, and I am betting it wouldn't be too good for the Ztron either.

Summary:
Both timers offer lots of functionality
Prices are very reasonable ~$30 --summer 2007
Programming of the JMP2 is fussy and analog---tough to do at the field or if under any pressure like at a contest!
Programming the Ztron is more straightforward, but you can still goof up (see my head scratching picture in the E-Nobler thread above).
3 Pin connector is just asking to be put on wrong---then poof!
Both could use more throttle click granularity to make setting up rpm steps for Governor mode on ESC smaller (less change of rpm per throttle click.