For what it's worth, these clones are emulation-based so GBA games don't run perfectly; you're not exactly getting a cheap AGS-101, basically they have similar capabilities to your average Chinese MP4 player/emulator console, only with the added bonus of a GBA cart slot.

There is one in the works (from "K-Team") that is an actual hardware clone, though, using a CPU which is 100% compatible with the GBA's when underclocked, and it seems to have perfect GBA compatibility; theyre also working on a version that can also play media and emulated games from an SD card. there's quite a bit of info here: http://obscurehandhelds.wordpress.co...-live-the-gba/
It can link with real GBAs too - the only feature it lacks from the original is GB/GBC support (apparently the technology supports it but the team felt there wasn't the demand to include it. Which is a shame, really - I'd buy one in a heartbeat if it could play GBC)