Well so far I've bought Super Mario 64, Sonic, Ristar and F-Zero. First off, the prices (as I'm sure you know) are very expensive. That aside, the service seems to be great. All four of those games seem to be spot-on perfect (as far as I can tell).
In terms of how they compare to unofficial emulators, well, Mario64 is a little different as PC N64 emulators are a little off sometimes. I've had N64 emulators improve on quality but not actually get things to look exactly as they did on my actual N64. The VC N64 emulation seems exactly as it was back in the day.
SNES and Genesis (MD) emulation, again, is exactly how it was. Having said that, if you're used to PC emulation, you're probably used to a clearer picture than what actually represents the real thing. Bear in mind I'm still on the composite cable and that affects picture but the picture quality is a little fuzzy compared with PC emulators.
So far, I've noticed no difference in sound.
I guess the big difference, for me, is that the VC is nice and official. I love emulation and it lets me play games I otherwise wouldn't. But I'll jump at the chance to do it officially. Prices, however, are too high so I can't see me buying a huge amount right now.