My experience was that everything was working smoothly with a fresh install of 11.10 - open source drivers, gnome shell or unity, the works - pretty much all worked as expected.. but now after this morning when I did an apt-get update / upgrade, everything got REALLY slow.
Unfortunately, i haven't had any luck with the proprietary drivers at all - they're showing major artifacts with transparency and font rendering and the HDMI out doesn't even work properly at all.
Good luck to you, I actually sent my M6600 back to Dell. The major problem for me was that they could not source IPS panels and their default 1080 panel was by far the worst quality screen I had ever used.
Weird, I wonder if you got a dud.. I have the default 1080p matte screen and I think it's fantastic. Amazing color, great brightness, excellent sharpness.. only minor complaint is their use of heavy anti-glare coating, which can cause some 'shimmering' when looking at a screen of mostly bright colors.. but I try not to do that anyway since I think it's a pain on the eyes.. I make heavy use of the negative window color function in compiz to mitigate that. Also, the anti-glare properties it provides far outweigh the small complaint of the shimmer for me.. I can't stand seeing reflections and glares when trying to look at the screen.
Also, turns out the problem I was having with performance was a poorly-made GTK 3.0 theme.. once I switched themes my problems with performance on the open-source drivers went away. Overall I love this laptop and ubuntu 11.10 on it.