Hi all,
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
SoundBlaster X-Fi
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.

The multiple dosbox installs has a purpose though - it guaruntees the game will work. If they had one install of dosbox that was updated as nessesary it would mean major issues. What if a new version changes behaviour and breaks a config? I say leave them as is, since it's a microscopic amount of space we are talking here.
As for the performance you should be getting better than that. First off to make it fullscreen you might want to try what usually works for me - set the output to 'ddraw' and then the fullresolution to your native screen resolution. That should stretch to whatever it is given.
Performance wise you could try settings cycles to "max". It should make Descent run better. I can't say anything about the resolution as I haven't tried 2 yet, and I never owned the original.