Vista
I run Vista on my desktop alone as a test for the office (bought a new PC, Vista was pre-installed, orders from above that we'll be moving to it anyway within a year so I figured a test was a good plan).
Out of the box, it ran like a sack of crap. Took ages to boot, ages to log into the network and indeed do anything network related. Applied every update that was available, improved things dramatically though network performance was still painful. Then I did some tweaking with some things I'd found on the net, disabling the global autotuning. It was perfect after that. Applied SP1, slow network performance, it had changed the setting back.
I must admit that, on this new £300 dell (2.8GHz Dual Core) I find I have no problems with it at all other than the odd IE lockup which I'm still trying to resolve. Certainly not all the issues everyone else has been having.
I guess the key would seem to be that it depends very much on setup, infrastructure, programs that are installed etc. I mean everyone here will be running different setups - it might well work faultlessly out the box for some people, others like me it'll work with a little tuning, and for some people it clearly doesn't work at all.
I think what I'm trying to say is don't assume because something isn't working for you that it must be broken for everyone and anyone who says otherwise is lying/working for MS. I mean I'm not sat here saying it doesn't work for you because you're a numpty am I? ;)