I've held off for a long time, running XP Pro as my main desktop OS and using Vista on occasion (dual-boot), also using Vista Premium as a main OS on a 1GB work laptop for over a year and still not being entirely convinced - I finally decided to move to Vista on the desktop last week. Frustrations with UAC mitigated with Norton's surprisingly handy and lightweight UAC tool and TweakUAC, plenty of issues with permissions on many of my own files, sorted using the command line "takeown" command (you may need to use icacls too if you can't solve things entirely with "takeown"), a couple of problems with some files not being indexed (tick the indexing box in file properties, never mattered with XP's hopeless indexing support, but it does with Vista). I can't say I prefer it to XP, the search is better when you've tweaked it to index the places you want (though I always relied on AgentRansack to actually find things anyway) and the improved security is probably beneficial, though turning off UAC and losing the Protected Mode of IE7 is not an issue for me as I use Firefox almost exclusively. I think it's worth the time to get used to it, but it's nothing like the jump from Windows 98 SE to XP - most of the features in Vista can be obtained with third-party software in XP, it tries very hard to stop you accessing areas that are not exactly security-risks (Start Orb/type `sendto` - not allowed access), gives you multiple UAC warnings for trivial activities like moving a few things around in the Start Menu and adds more steps to ordinary activities like accessing network and sharing configuration for no apparent gain.
To sum up, turn off all the fancy graphics (I'm running in admittedly dull-looking Windows Classic theme), tweak UAC and search indexing, get used to running the command prompt as adminstrator all the time and you'll have removed many of the irritations. It's not the OS it should have been (and don't even bother if you have less than 2GB of RAM) but it does have some useful modernisations compared to XP - built in media centre for example (unless you have Vista Basic) and much better picture/movie/audio management.
First post for me, so I'm including links last;
Norton UAC; User Access Control - Vista User Account Control - UAC | Norton Labs
TweakUAC; Disable annoying Vista UAC popups with TweakUAC (FREE)
Add index locations; Adding Indexing Locations in Windows Vista :: the How-To Geek
Takeown/icacls (but not for deleting files) ; Tim Sneath : Windows Vista Secret #11: Deleting the Undeletable
I must mention that I did an in-place upgrade from XP Pro to Vista which is something I would normally avoid, so you may not suffer the issues I have with a clean Vista install.
HTH