I've been using the lithium ions since they became available, like them very much, and haven't experienced problems and don't know about any "issues". They do require a regulator though. That presents a small weight and wiring thing that some people don't like or want to deal with.

Nimh or A123? I suppose that depends on what radio you have. 2.4 you might want to seriously consider the A123. They hold a higher voltage longer and can be fast charged at the field. If a 2.4 gig radio drops to a low enough voltage you have big problems. Don't telll anyone but the same thing happens with any other type of radio so there's no excuse for not paying attention to battery levels.

You really can't fast charge a nimh. You can fast charge a nicad if you have the better cells. If you have a 72mhz radio then the nimh might be more to your liking. You probably won't fly enough flights in the normal day to run them down to the level they need re-charged if you have 2,400 mAh or greater. If you have a 72mhz 9303 tx with the standard battery you might want to think about a 2,500 mAh nimh replacement pack for it. The standard 600mAh tx battery isn't worth a crap for a serious day of flying unless you have a field charger.

Rather than go to TBM about batteries, why not talk directly to Tom? He's done a lot of research on what works best in different situations and has a solid handle on what works best in his planes with different radios and servos. He sells just about all of them set up and ready to go.