I have a portable one that attaches to the GBA SP that is pretty sweet (can't remember the name). I'll try to post photos or figure out the name later.
I have one like that called the Handi FamiEight (sic). I'm assuming the core functionality is the same as the FC Cassette since it's plugged into the GBA slot on that SP.
The actual image you get on the screen is...not good. Its basically f*cking terrible, honestly. Its not un-playable, but its pretty awful. Everything else about the unit is pretty good though. The composite out is damned good. It's cart port is native FC but comes with a NES adaptor. It also comes with two controllers that aren't that bad by the standards of Famiclones.