In my opinion, two things make or break a handheld (not for the market so much as for my enjoyment; though I maintain these are still the biggest two issues as to any rating it should deserve).
Not the battery life (heck, I'll love a system even if it's tethered to a wall; that just cuts into the play time I can have with it).
Not its weight or size - grow some muscles on yer wrists and forearms, it's good exercise!
No, the game selection and the screen.
I must say, now I know why the Game Gear failed: The screen is horrible (the Super Wide Gear fixes this). Horrible horrible; it's like an RF connection that you can't fix. The Nomad - as I remember - was notably better, partially due to the higher resolution. The Game Boy Pocket's screen was great; even though it was grayscale you could see it with ease and that's worth more to me than some splotches of color.
As for the geme selection, ugh, all the Game Gear platformers I've played in real life were pretty bad reminders of why I'm glad this era is over. Scaled-down Lion King and a Simpsons (versus The World, I vaguely remember) game, as well as a pretty weak entry into the Mickey Mouse platformer selection with a flickscreen Land of Illusion. Actually, with the exception of the import title I bought a while back, I don't have any good games for the system and I have 7 unique ones :x