I wouldn't buy Vista to upgrade a computer that's fully capable of running it, but when I got a new laptop to replace one that died it came with Vista Home Premium. I like it, though I'm not blown away.
It's not slow at all, it boots more slowly than XP (though apples to oranges--that's another computer), it shuts down faster than XP, it has been completely stable in the 6 weeks or so I've been using it, and I've found it pretty easy to set it up to my liking.
If someone's Vista is booting slowly, it could be a hardware problem even with a new computer (premature failure happens early on). Probably, though, it has too much stuff loading in the background, as is typical with new computers that come with preloaded software, trialware, crapware, excessive antivirus and antispyware apps, third party firewalls, etc. I've seen computers with 50 entries in MSCONFIG's Startup tab! Anything Norton on a system seems to have a molasses effect on it, and I've heard the same about McAfee.
Any computer running Vista with less than 2 gigs of RAM shouldn't be using Aero effects, particularly transparency. Actually, I think 2 gigs is a sweet spot; 1 gig isn't enough, though it'll do if you don't run much at the same time and don't use much eye candy.
I've turned off UAC with its pesky questions about whether I really want to copy a file. In theory, UAC is a good idea (it would prevent drive-by and other malware activity). In practice, it is poorly implemented. I do use antivirus in the background but no antispyware or other such stuff. They slow down a system and aren't needed if you have safe computing habits. Well, I need to modify that--I've left Windows Defender running and it doesn't seem to make much of a hit on the system.
I've also turned off indexing. It's supposed to work when nothing else is active, but I hate seeing my hard drive flickering all the time. I've never done much searching anyway, as I have a neat and well-organized system and pretty much know where things are.
I don't like the Classic Start menu. At first I found the Vista Start menu to be a mess, but a bit of exploration showed that I could tame it quite nicely and it's now efficient. If you don't feel like working your way through the Start menu, just type in the Search box the first few letters of the program you want to start and it'll show up quickly.
The wireless and the sound on this laptop basically don't work with Linux on its own partition at this time, but I'm writing this from Opera in VectorLinux 5.8 Standard running in VirtualBox under Vista. Works a treat! Thanks to the hardware being virtualized, everything is working and very happy in the virtual machine. One of the glories of new hardware is that we finally have enough power to run virtual machines, which sure do beat the pants off dual boots.