there are 2 advantages -
1: they wont blow up/catch fire like a mistreated LiPo
2: you can recharge them quickly, and in the plane 

1:disadvatages - they are HEAVY
2:you need to wire up your own pack, including balance lead
3:you need to work out a different system to the LiPo. see Igor Burger setup
4: they are HEAVY
5: the one being sold by Hobbycity are NOT A123 packs
6: they are HEAVY
7: you will need 6 or 8 batteries to get the correct voltage and each cell is heavy
8: you cant change the capacity of the pack - as you need to wire them in series to get the right voltage- but you cant wire them in series AND parrallel to get the right voltage AND capacity

did I mention they are heavy? 

The most important point is, that hey can be built in the fuselage ... because:
 
1/ they are not kids baloons
2/ they almost do not have looong CV phase at charging, so they can really be charged in 15 mintutes (unlike lipo which can be charged at high current, but full capacity is recharged after long CV phase)
3/ they not need balancer
4/ they work well HOT
5/ I do not know about any failed cell yet, the battery after 500-600 flights lost ony 100mah of capacity

The weight disadvantage is not so bad, if you consider they can be discharged to 0 capacity, I even know flyer who do not have timer, he just fly until ESC cuts off :-)))

The lower voltage makes 6cell pack equivalent aproximately to 5cell lipo (A123 is little higher).

They have only one capacity, so smal adjustments can be done by number of cell in serie, not big problem, but it can in some cases lead to motor of different KV.

Old cells can be former to 4s2p blocks what makes 12V "fuel cans" which have equivalen charging and discharging voltage as and 12V lead acid battery 

I am from the Ni-Cad generation.  Ni-Cads also can be drawn to ZERO however they have a drop in voltage during the discharge.  If LiFe have the same voltage drop headroom at the end of the run could be a problem.  LiPos at 80% still have a good voltage level - ??  Don't Know just food for thought.