can anyone confirm the problem with mouse aiming?
Rebellion made the game. Rebellion claim it's patched to work on win7 and vista. Steam is irrelevant.
By the by, of the 4 GOG games I've played so far, I've had problems with 3. (graphics and crashes for Messiah. Speed issues for BG&E and Painkiller). But then again, even when buying a brand new Pc game, compatibility is never guaranteed.

I only played the first Marine mission and first Predator mission so far but here are no mouse aiming problems so far.

Steam is pure awfulness GOG is brilliance the only GOG game I ever had a problem with was The Longest Journey though to be fair I was running it on Win7 64-bit which GOG does not offically support - yet...
Oh and steam dosen't seem to trust you when you but their games and fill your game full of DRM junk.

Yup, I have had a few problems with GoG games too. Hell, Arx is still unstable as hell (albeit, more stable than it was on Steam :p). It is a mixture of Steam having a MUCH larger userbase (so more people who get errors), but also Steam having lower quality control. Steam just needs to make sure the overlay works. GoG goes the extra mile and makes sure the game works (more or less).
As it stands, the AvP bugs seem to be pretty hit and miss. Some people have massive bugs, others have no problems at all. I would actually suggest those having problems detail what they are doing to launch the games. It is actually somewhat conceivable that they might be running the wrong executable. I know that the GoG version of Die By The Sword had a similar issue, where people who launched from the Vista Games Explorer complained that the game was buggy and lacked the expansion pack, whereas those who launched from the shortcut (or had a different games explorer link) had no problems.