I think that the Li-Ion work great and I agree totally with Tom on this one.
My set up on my 35% Panzl.
2 TBM Li-Ion 2600mah batteries
2 JR Heavy Duty switches
2 MPI 6V regulators
This plane has 8 JR 8611A's on the control surfaces and 1 Hitec 5645 on the throttle.
I can fly 5 fights without recharging. I could probably make 6 or 7 flights but after the 5th flight the batteries read in the range of 7.5V under a 1 amp load.

Just remember that your trusty voltmeter that you typically use on smaller planes for Ni-Cd's and Ni-Mh's will not give you the correct reading. It does not inject enough load in to the circuit to give you a correct evaluation of your battery voltage.

The setup that I will be using on my Sukhoi is the same as the one above that I use in my Panzl. Two batteries, two switches, two regulators. The only difference will be instead of 8 8611A's there will only be 5 on this plane.

Always check you batteries with a voltmeter that can load the circuit before every flight. It will save your plane.