I changed the plug in my Time Machine after 241 flights the first time, then just changed it again at flight 560 this past Sunday.  It makes a big difference in how the engine runs, now I have to go back to my logbook to see what nitro and needle setting I went to last time to get a good 4-2-4 run.  After 200 runs the motor run starts to sound a bit flat, and sometimes it makes a pop like a backfire in level flight.  I find I keep going up and up in nitro level on the very old plugs, and a bit in on the needle, but the engine stays in a rock steady 4-2-4.  Finally, after running great for so long,  it becomes unresponsive to the fuel and needle changes, plus my flying buddies start to cringe, saying they can hear the motor running rough out there.  Putting in a new plug really makes the engine crank, it seems to need alot less nitro and is a bit jumpy on the break for awhile-its easy to miss a setting and end up a teensy bit lean, compared to the old plugs where its almost impossible to make it too lean.
  This is a Sig 003 plug, I'm wondering if the heat range is a bit off on new plugs (too hot or too cold?) I'd love to find a plug that duplicates the fat rock steady 4-2-4 of an "aged" plug without running the risk of the engine quitting when the (old) plug finally dies.
  Oh, the motor is a DS-60 lite with 4 0.010 head shims, running between 12% nitro (new plug) and around 14% nitro (old plug).

Aside from the typical RPM drop when you pull off the battery, which is more noticeable on 4-2 break settings than high RPM runs. On 4-2 break setups I do tend to notice inverted flight gets leaner on a tired plug also.

On the higher RPM setups, I notice that the run gets ratty. Burps or moments of silence during the pattern usually get my attention! 

Pulled a plug last week that was doing this on a piped PA61 and found a fair bit of carbon build up.
Changed the plug and the RPM came up quite a bit, like 700 RPM. So I turned the needle out to what I had been setting it at recently (9300) and got 5.8 second laps! Had to turn it back in to work with a new RPM range, (at 9650 I think?) to get the same lap times as the old tired plug (5.3 laps)

It was an Enya #3 and only had about 150 flights on it.

EricV