The battery life is short. maybe no hour and a half short, but even at 5 or 6 hours, its still less than the 10 DS and SP offer with the light on, not to mention to lengthen the battery life on PSP, you have to change brightness settings or volume. That or buy a peripheral battery, which is a pain. If you played the PSP at max settings it the battery would tap out fast.
Personally, this is why I think cartridges are better for handhelds. No load times, no disc accessing eating up the battery life and best of all, the responsibility of maximizing battery life isn't left in the hands of developers and consumers. Yeah, the visuals and size of games take a hit, but handhelds are all about the quick fix anyway.