The Lithium polymer AA/AAA/etc batteries with the built in USB charger are neat. It is slightly annoying that you can only charge them via USB, due to the higher voltage requirements of the actual LiPo cell. Both rated cycles and subjective data (reviews) seem to indicate durability isn't as good as decent NiMH batteries (Eneloops or similar). But for most usage probably still good enough.
Also they're not exactly the same size as standard batteries... some devices may have issues with fitment, as these are longer than standard AAAs. I imagine the AAs suffer from similar issues. So you have to keep that in mind. The mAh rating in many rechargeable lithium batteries is also questionable, but these converter-equipped batteries do output 1.5V - unlike NiMH competition. That's a huge coup for devices that have issues with lower-voltage NiMH batteries, which previously might have only run well with disposables.
Anyway I wish more companies would go the route Kentli does with their Li-ion AA/AAA batteries... they added a recessed ring so you have a 3.7V terminal for charging as well as the 1.5V output terminal. I'm not sure how GOOD the Kentli batteries actually are, and they are not priced competitively, but the concept is really excellent.