I had read so many negative reviews, with the positive ones usually seeming to be by ones that didn't truly know the old hardware or hadn't grown up with it (casual sorts) and the trashers being purists who loved the system(s) as a child... however I loved the NES growing up and the emulation/NOAC (or whatever it is) is surprisingly good! The colors are rich (really!) and the black levels are good (night in Contra and Journey to Silius looks pitch black). It's probably over-saturated by NES standards, but I like the look.
Sound is another story. There is an ugly twang at times in the music, but not enough to keep me from enjoying a game. The controllers feel nicely clicky for the most part, but playing through Contra there were several times I died simply because the d-pad wouldn't press down easily enough (it's too stiff -- perhaps it'll break in).
But for $40 it's a nice buy, and nice to buy new hardware for old games rather than rely on aging stuff from someone's basement. I recommend it... but not yet sold on Gen-X/FC3 (or whatever) that has the Genesis. From what I saw on YouTube the quality just isn't there yet.