I switched a few weeks ago and I really like Vista. I don't think it is worth the money if you have XP, but I think it is definitely faster and easily more stable. I've expereinced almost zero in the way of crashes or compatibility issues. I especially like the update system.
Vista has some great ideas, but I still feel it's "not quite there yet." In my experience it is way less stable than XP; Explorer keeps crashing (it doesn't like you to visit certain folders), it has a few minor issues with some applications too.
I have a few cosmetic issues with it as well: I can't stand the new start menu.. I hate scrolling in menus and I don't want the Classic start menu, I just want the XP start menu but that's not an option.. Or at least the Vista start menu but with an "All Programs" menu like XP or Classic.
One more thing is the icons. I like the 32x32 icon size but that's only am option on the Desktop by selecting "Classic icons" in the view menu. It is not an option in folders or Explorer view however. Icons are either too big or too small. I don't realyl understand why they'd remove the option. It's not exactly complicated.
The other cosmetic issue is wallpapers.. They added a few cool features but they apparently removed some too. For example, if an image is larger than your display you can no longer Centre it. It has to be resized (with proportion), stretched, or tiled.
I guess Vista is only worthwhile if you buy a new computer and it happens to be on it, otherwise it's just not worth it.