Search from Start Menu in Vista is far far more productive than the old "search for your app somewhere" method of doing things in Vista, I can probably launch my every day apps (and most of the hard to find stuff that would be menu's and windows deep) also in a couple of seconds because of it. In that way Vista is leaps and bounds more productive than XP or any preceding OS from MS.
The DRM thing, I have no idea what people are going on about, most people seem to believe that it's somehow a method for stopping you playing un-DRM'd content, what absolute rubbish - and can only come from people who have never actually used the OS and tried.
Vista's UI is quite attractive (though I felt the original Longhorn designs were FAR cooler than what Vista has now) and I find the new explorer layout to be quite intuitive myself, definitely allows me to show more content (or less if I want) in an easier fashion than XP did, and having access to common folders at all times on the left bar makes a lot of sense and allows me to get to my user folders faster than XP did in the past.
UAC I feel, while irritating when first installing your box, is a welcome addition and the only times I get UAC prompts now is when either a) I run an app designed for previous versions of Windows that inexplicably require admin access (when they shouldn't and don't) or b) install something, which is not *that* frequent so as to get irritating. The frequency in which I get the popup is very low now as my installation has begun to settle.
Finally there are some kernel level changes which are quite important, such as Superfetch for improving application load times, improved threading, better use of memory (of which Superfetch is a part) and some non-kernel changes which are pretty welcome, such as a "new and improved" network stack - though I must say I've seen a couple of problems with it that have been a bit irritating, and of course the desktop window manager (dwm) which provides you with those nice desktop effects, which while not essential, definitely adds to the whole experience - and for developers who want to extend the system it's far better than XP (and previous Windows version's) GDI-based UI.
That said, Vista isn't perfect, there are some issues:
1) Sometimes if I resize the task bar the quick launch icons jump up by probably 12 pixels, resulting in them being offset incorrectly... can't believe none of the testers picked up on it, or maybe it's just on my machine.
2) The most irritating thing about the (now infrequent) UAC prompts is the screen blanking, I can't believe MS finds that acceptable - there *must* have been a better way to do it.
3) nVidia's drivers are pretty damn sucky atm, so the animated desktop stuff *apparently* takes more CPU than it needs to, but that's more a nVidia problem, not MS.
4) Some apps just don't work, but that list is shrinking now with compatible updates coming out, thank gawd.
5) The new Media Center stuff is okay, but some of it feels like a huge downgrade in usability compared to MCE 2005. Also it's partially broken my ability to transcode videos to the XBOX 360 so I'm waiting for a more stable update to the transcoder to come out.
Is Vista an "ESSENTIAL WINDOWS UPDATE!!!11!!1" no, not really, XP is going to do you fine for quite a while, but while it's not *essential* to update to Vista, there are lots of compelling reasons to do so, for security I would absolutely recommend upgrading to Vista, it is just more secure than previous Windows versions, full stop. If you have a beast of a machine, and want to ultimately get all you can out of it (including a more efficient OS), I would say Vista is probably also a good idea. Note however when I say the OS is more efficient and you want to get all you can out of it, that doesn't mean I'm talking more FPS in games, it means that overall the OS is better at scheduling threads to other applications, arguably better at running media applications, and generally more responsive than previous versions of Windows.