I bought my Sega Nomad via eBay a few years back and haven't regretted it for one moment.
Yep, the battery life is atrocious but, in common with my Lynx and Game Gear, I play it off the mains anyway.
It may be a bit ungainly and not exactly the most ergonomic handheld device out there but other than that I rate it highly. The screen, although understandably not great by todays standards, is nice enough and certainly doesn't have the godawful motion blur which blighted Game Gear. I'm aware that it's possible to replace the screen with a better more modern one but have resisted the temptation so far for various reasons.
The d-pad is outrageously good and the six action buttons are responsive and well placed. I've not yet played on Nomad in two-player mode but the inclusion of the controller port so to be able to plug in a standard Mega Drive controller was a stroke of genius, imho. Two-player gaming might be a bit cramped when viewed on the system's own screen but when on a TV screen it'd be just the same as using a Mega Drive (or Genesis, to be more correct).
One real highlight, for me at least, is that when hooking Nomad up to my TV screen (via the same RGB SCART lead I used with my Mega Drive II) I can play games at full speed/60Hz. Going back to playing the likes of Sonic The Hedgehog
at 50Hz on my PAL Mega Drive is now a painful experience so I now use Nomad to play all my compatible Mega Drive cartridges and only my Mega Drive when wanting to play Mega-CD and/or 32X games.
Own: Jaguar JaguarCD Lynx 7800 Dreamcast Saturn MegaDrive MegaCD 32X Nomad GameGear PS3 PS PSP WiiU Wii GameCube N64 DS GBm GBA GBC GBP GB Virtual Boy Xbox Vectrex PCE Duo-R 3DO CDi CD32 GX4000 WonderSwan NGPC Gizmondo ColecoVision iPhone PC Mac