Looking around the game specific forums gives a bit of a negative sense as to how GOG is doing on that front, but naturally people whose games work fine won't be posting there.
That's the point, and even with GOG games it is the same as PC gaming overall: a game that works just fine on some system does not run at all in some other.
Most GOG games I've tried so far have worked fine from the get go, at least on the system where I initially tried it. But then if I take something like Baldur's Gate or Icewind Dale: on my ATI/AMD powered PC they work perfectly, on my NVidia Geforce machine I had the "jaggy edges on fog of war"-problem that seems to be quite normal with the Infinity engine RPGs when run on modern NVidia chipsets/drivers. You can get around that problem by changing some graphics options inside the games (software BLT and something else...).
The GOG games where I recall having the most problems were:
- Dungeon Keeper 2 3D HW acceleration failing to work on AMD/ATI graphics hardware. GOG fixed this later, now it seems to work fine.
- Gorky 17: This still seems to have problems mostly on NVidia hardware, when you try to run with 3D acceleration. The workaround is to switch off 3D acceleration (which makes the game run on slightly worse graphics), but it seems on some Win7 machines this may cause scratchy sound.
I played it through in a ATI/AMD laptop where it worked fine with 3D acceleration and good sounds, but even there one late mission kept crashing at the same place. The workaround was to switch off 3D acceleration for that one mission, and switch it back on after you finished that mission.
- Shattered Steel: By default the laser type weapons miss your targets all the time, as in just going through them without causing damage. That seems to be some CPU speed problem, because slowing down the CPU cycles inside DOSBox (Shift-F12 or something) fixed it. You just have to find a suitable speed for your PC where the problem does not happen anymore, but the game still runs fast enough to be playably smooth.
- Interstate'76: This is probably the hardest case, it has some bugs related to higher CPU speeds. In the Interstate subarea there are instructions how to try to slow down your PC with a third-party program, and that fixed the physics modelling of the cars (the faulty physics modelling mostly made the game _too easy_, as all other cars except yours would run quite slowly; so in essence the fix made the game challenging again).
But even then the other bug with the flamethrower type weapons not working correctly remained, ie. they wouldn't reach more than a meter or so from your car, making them useless. You don't need those type of weapons to finish the game, but of course it is not nice not to be able to use the whole weapon arsenal. The only time I could get that problem to disappear was to run the game in an old IBM ThinkPad T40 laptop (running WinXP), and even there to slow down the CPU speed way down.
Not a GOG game, but I also bought Saints Row 2 from Steam, anticipating lots of problems since there are lots of complaints about it. Something to do with the game expecting your CPU speed to be exactly the same as with XBox360.
Anyway, for me it seems to run fine on my ASUS G75VW laptop, at least I haven't noticed anything abnormal yet (like the game becoming a slideshow during driving, or the driving model being far too twitchy, etc.). Maybe I'm lucky.