got on the beta a week ago and have already bought Fallout and Fallout 2, Descent 1 and 2 and Stonekeep - all great games and I'm particularly pleased to try out the Fallout games.
The browsing for games, purchasing and community forums are all great and I think the pricing policies, approach (or lack of) to DRM and additional freebies are terrific - I hope GOG succeeds as I've been wanting a service like this for a long time - good luck!
That said (you knew there was gonna be a 'but' :^) I'm not too impressed with the DOSBox functionality - my PC's not exactly new but it's not an awful spec either:
2 x Pentium 4 Xeon CPUs @ 3.06Ghz
4 Gb RAM
Windows Vista Ultimate x32
Sapphire Radeon HD 3850
Fallout 1 and 2 run great but the DOSBox games unfortunately have issues - the Descent games only run in 320 x 240 res (D2 has the option of running in 640 x 480 but it does nothing) with the video offset at the top of the screen (I've tried other renderers but each have their own issues) and Stonekeep's video/intro is horribly slow and out-of-sync. Not major issues but I'd expect better running these ancient games on this spec PC.
I know DOSBox has config options you can tinker with to get stuff running smoother/higher-res and I'll probably look into that but really, I was hoping to 'install and go' which isn't really what I'm getting. What I'd really like to see is GOG bundling a decent, user-friendly front end to DOSBox that allows you to get the best out of these games.
Oh, and can we get just one install of DOSBox? I don't see why each game you install that uses it needs to have its own copy - OK, it doesn't exactly take up much space but it seems silly to have multiple copies on your hard disk.