I've switched last christmas (7600GT -> HD5770), some bugs went away, other bugs appeared. Overall I'm glad I switched.
The biggest gripe with fglrx seems to be that it's problematic in two popular use cases: wine gaming and video playback.
I've resorted to a real windows for gaming (most of the newer titles I'd wanted the new GPU for wouldn't work on wine anyway) and I don't care about the issues with video playback: to my eyes every video I've watched looked good.
I just issued
and - behold! - my second monitor rotated as expected. I didn't do much long-term testing, but composition (kwin3) continued to work and glxgears ran at the same speed no matter which monitor it was on - even if it spans across both monitors.xrandr --output DFP4 --rotate right
#1: seems to work
#2: I'm on gentoo, I did 'emerge -C nvidia-drivers; emerge ati-drivers', and after slight adjustments to xorg.conf and the multiscreen-configuration (bye bye TwinView!) everything worked fine. There are reports of people having more trouble with driver installation, I have no clue why.
#3: scrolls snappy here, unless there's flash in there. You may want to use the backclear-patches for xorg, they'll speed up some operations like window resizing.
#4: define "decent"? There's no hardware decoding yet, but the 7800 GT didn't have that either. If you could watch the video before, you can still watch it now. The two main issues on fglrx are: slightly washed out colors with xv, tearing with xv. You can use opengl instead, but that'll eat more CPU.
#5: for wine, see above (although simple games like snes9x or VisualBoxAdvance work well here. GuildWars runs fast, but crashes after an hour or two.)
3D performance is actually very close to windows, if you're running native games/apps.