I have an A123 battery in my HP2. I like it a lot. It's not perfect. It's expensive. But it's super small and extremely light. My first one failed after about 18 months or so. It's not clear what caused the failure, but one of the cells ruptured and the battery couldn't hold a charge after that. As far as I can tell, failures seem to be pretty safe with these batteries.
The new A123 batteries come with load balancers so you can carefully check to see the battery is properly charged cell-by-cell every once and a while. You don't need to check very often because the charging from the alternator seems to be good enough most of the time.
Here's my install thread: http://advrider.com/forums/showthread.php?t=687012