I'll admit right away that I'm mainly a PC/Windows guy, with a pretty long experience of using Windows (since version 3.1). I thought I'd give you guys my view on Vista, after having used it as my main OS for a couple of months. First, let's get the good stuff out of the way:
The OS is indeed stable. No crashes or BSODs so far. I've gamed, rendered, ripped music, run stability tests, etc. without any OS crashes. Applications have stopped working or refused to start, but that's been apps designed for XP. Those can't be guaranteeed to work on Vista (many because of bad design), so you can't blame the OS there.
It's fast. The caching of data in RAM (Superfetch) really does work and it fills up my 4GB of RAM completely. What's more, it immediately frees up the RAM for apps when needed, so no complaint there.
It's not nearly as resource heavy as some people think. It does use more RAM than XP, but that's nothing to moan about. XP is, after all, more than 5 years old and Vista doesn't use that much more anyway. Other than that, neither a very fast CPU nor a fast graphics card are needed to run Vista with the "Aero" interface enabled. I've run it on an old Athlon64 3500+ with a very slow GeForce 6200 Turbo Cache graphics card and the only thing that ran a little slow was Flip3D. I find it amusing that so many seem to think that they need to upgrade their computer just to run Vista.
I can't say much about security yet. Vista supposedly has had a lot of work done "under the hood" and should be much less likely to get infected by malicious code. That doesn't mean I trust it, though. For what it's worth, after two months of use, my installation is still free of spyware and viruses and that's without running any antivirus software in the background as protection. Then again, my XP install has also been running unprotected since September of last year and is also uninfected, so I'm not sure if this tells us much.
The search function on the start menu is fast and works well. Can't really complain there.
Windows Update is finally integrated into the OS, which I consider a nice step forward.
Most other things that I like about Vista are little things here and there that are improved over XP. Nothing earth-shattering, but nice nontheless.
Okay, onto the bad:
As said before in this thread, the user interface is a complete mess. Windows XP might not have been visually stunning, but the UI worked very well and was consistent and fast to work with. Vista is the complete opposite. The control panel is particularly hard to use. Using Vista's default control panel view or classic view doesn't matter much, both are confusing in their own way. There simply is no consistency between the different setting windows. There seem to be a handful of different layouts used and many are cluttered with "useful" shortcuts that simply make the screen confusing.
Using folders and the file explorer is also confusing. Folders aren't handled as in XP, but instead double clicking a folder on the desktop seems to start up a file explorer view pointing at the chosen folder (complete with a file tree on the left). Disabling the file tree to get a more clean looking folder view is possible. The downside to this is that if you then enter the file explorer and click on a folder in the tree on the left, the tree view will get disabled... So, it seems folders and the file explorer are one and the same now. In my opinion not a good decision.
Another bad decision is that regular menus (File, View, Help, etc.) aren't enabled by default. Microsoft chose to use newbie friendly icon based bars at the top of the window instead. It's limiting the choices that can be made and makes more advanced settings harder to reach when needed.
UAC sucks so much. It destroys the computer experience and can't be left enabled.
That's pretty much it. The problems I have with Vista are almost exclusively UI related, but that is a pretty major thing about an OS. Because of these UI problems, I'd have to consider Vista a failure (atleast for the time being). It makes so many mistakes compared to XP and that's simply unacceptable.
Finally, I have to address kind of a pseudo problem that other people have written about and that is the apparent instability of Vista. Vista isn't unstable. The problems people have are caused by bad drivers and incompatible applications. These are issues that can't really be blamed on Vista. It's like all the people complaining about XP BSODing, when it's almost exclusively caused by bad hardware or drivers. Microsoft is an easy target to attack, but they're not responsible for every little thing that goes wrong on your computer. Believe it or not.