I'm an enormous fan of Vista, but then I set up clean-installed systems on known good hardware and intentionally kept them clean.
The new security model with the split/low rights features is outstanding, and general stability and the excellent new diagnostics features are good. Vista's use of ipv6 is commendable, so too are performance improvements in SMB/CIFS. The new shell/GUI is sluggish (presumably XAML rendering) but better in SP1 onwards. File and Registry Redirection are life-savers.
The only really really bad things i've encountered are
(1) Vista PRINTING - a complete fiasco if using old printer drivers, and no-one can tell you what "old" means. Thanks to HP for going out of their way making a catastrophic global farce with drivers that never needed to exist. Useless, cynical, lazy crooks.
(2) The bad router fiasco, where cheap SOHO routers mangle traffic which uses TCP window scaling. Hardly Microsoft's fault, but they should have (i) had a war on duff routers before the release of Vista, after all, then had over five years to lean on mfrs to get them to sort this out, and (ii) should have put something into the product to detect the problem (which I believe is now the case post SP1? SP2? with the "heuristics" thing, poorly documented and mysterious as it is
(3) Bizarre UI with non-SSID-broadcast WLANs - very weird and nasty UI behaviour.
Windows 7 seems nice and snappy. Printing is still a mysterious illogical mess though, incredibly.