I installed the Vista SP1 beta a few weeks ago on Vista Business 64.
It does seem to be more stable but this isn't the "end all" of service packs like NTsp4, 98se, or XPsp2 were.
Personally (running Vista on 3 PCs and XP on 4), I haven't experienced all the issues that most people say. Most of the "hardcore" review/benchmark sites do not understand what Vista does. You cannot do a direct comparison between the teo because they are completely different Operating systems built upon different architectures.
IMO, if you are someone who doesn't like your Computer doing stuff for you (that you didn't ask it to do), then you are not going to ever be happy with Vista. If you listen to the "experts" and disable those features, then you cripple the OS and it is going to lose to XP 95% of the time.
On the other hand, if you are willing to put a bit of trust in the hands of company that built the OS, you will find that it is overall a pleasent computing experience.
If you are buying all fresh, there is really no reason to not go Vista at this point. And let vista do it's thing. You will see higher CPU and Memory usage during normal operation (as well as when idle) but you will not see reduced overall performance in your apps.
On the other hand, if you are planning on just buying a tower and hooking up your existing printer, keyboard, mouse, microphone, webcam, etc you might run into problems and I would recommend XP.
You also want to take a look at the apps that you run. If you have a lot of "legacy" software that you run becuase you can't or don't want to upgrade to new versions, you might end up having problems with vista.
On the other hand, if you are holding out on the Vista change for hardware or software issues, it is certainly time that you start thinking about "getting with the times"....
Vista IS the future of Operating Systems. Simple as that. You will have to update eventually unless you never plan on needing anything more out of your computer than you have now and never plan on your current computer breaking down.