Lithium Ions are great batteries. Unfortunately, it's my understanding they're not yet available in a 1.5 volt, AA size cell. Please correct me if I'm wrong.
I like how my new Fenix flashlight will accept any AA size cell which puts out anywhere from 1-4 volts. So I can use alkalines, NiMHs, or 3v lithiums and 3.6v Li-Ions (but I don't have a charger for the latter yet). It's really nice to have this flexibilty, and I wish it applied to more devices out there right now.
I do not want to become a slave to custom Lithium Ion packs and no other option, which I why I seldom buy any portable device that won't accept AA or AAA batteries (especially for caving).