Ok, I've installed the bugger... (Umm, bugger? This is actually an interesting attribute for anything related to software development :))
First try: installation on a "clean" machine (no SP1 beta, no CTPs and the like). Forced me to uninstall Web Application Projects add-in: the installation package claims the add-in is included in the SP (although there have been controversies about this on the net). The installation takes a bit more than an hour. I don't remember how much it took to install Visual Studio, but compared to SP1 beta's 3-4 hours, this is nothing.
Upon first installation, I tried randomly opening several forms, controls and our components with our own custom-made designers. Either I was (un)lucky or Visual Studio really started behaving much better, but I couldn't get any of the errors I kept getting earlier (like "path is not a legal form"). I did find one bug from earlier though, and it was the first one I tried: if you try to "Open containing folder"while you're in a exception-breakpoint, Visual Studio will freeze until you force-close your application.
Second try, over a SP1 beta installation. Well, it's also not true that you have to uninstall SP beta, at least on Windows XP. At least in my case... Once again, no problems.
As for the quality of the service pack, it seems satisfactory. Visual Studio does seem to run more smoothly. I haven't experienced any slowdowns, component loading errors don't pop up as often etc. I even noticed that marking of container foreach statements when you type "break;" into the C# editor once again works.
So, the conclusion? Visual Studio works better with SP1 than without. It almost seems to behave nicely, but it's early to say. In any case - recommended.