Sorry for the wishy-washy answer, but... it depends. Its main advantages are its low cost, long battery life and better video playback than the GPH consoles. It emulates the GBA, NES and Megadrive pretty much perfectly (Megadrive needs Dingux + Picodrive). The SNES emulation isn't as good as the other consoles but plays a lot of games okay with a little frameskip. I haven't played around with MAME yet (too much like hard work IMO). There's not really anything a Dingoo can do that another console can't, apart from some homebrew, but it's pretty capable for what it is.
Personally, compared to the bulky GP2X Caanoo, I find its small size and long battery life is nice to have when you're out and about. Because the headphone socket is on the side, you can also put it in your pocket and use it as an MP3 player (a fairly basic MP3 player but still). I got mine for around £40/$60 on eBay though, I don't think I could have justified spending $100 when Thinkgeek is selling the (superior, but equally small) Wiz for only $99.
I don't know much about the Gemei, I'm afraid. Most of the Dingoo's best stuff was on Dingux, and the Gemei doesn't have Dingux yet, but the guy is working on it (with support from ChinaChip!
Well, sort of), so who knows?