I love my Nomad. I love pulling it out to play a Sega game and it gets good use. I have the Sega Ni-MH rechargeable battery pack and would recommend it to any one that is thinking about picking up a Nomad. I have not yet tried any of the modern AA rechargeable Ni-HM batteries in my Nomad, so I have no idea as to the playtime that you get on those. Past estimates of battery life were determined on regular alkaline AAs, I think, which had very poor lifespan on the Nomad.
jb143 have you determined what the playtime of those AA Ni-HM batteries are giving you?
Do be aware that holding the Nomad in your hands with any of the battery packs and game cart is like holding a brick that has these odd worts.
One good thing about the Nomad is that you can hook it up to a TV and play, and if you have an extra controller, play with a friend.