ITWire has done a hatchet job on Windows Vista. Writing about their experiences on a brand new Lenovo laptop, with Vista pre-loaded, the experience they describe is horrid. Slow, manufacturer delivered utilities that are uncertified, and with "new functionality" that doesn't seem any better than the XP functionality it replaced, they're unimpressed.
My experience was different but similar. To begin, I didn't purchase a new PC. I upgraded an existing one. I experienced all the same problems with my formerly XP desktop once I had put Vista on it. It was slow, a lot of my existing software triggered failures and alerts, and the user interface was unfamiliar.
To deal with the slowness, I simply added RAM, and I plugged a 1G USB key into a spare USB port to use as a "Ready Boost" disk cache. The USB key essentially caches part of the swap file in fast non-volatile memory. The impact of the USB key was that Vista didn't page as much as before on loading applications that I use frequently. It was dramatic. Moving from 512M to 1G RAM turned an unusable PC into a sluggish PC. Moving from 1G to 2G was a huge improvement — sluggish to snappy. Memory makes a huge difference. My aging three year old Toshiba Notebook, with 1.5G of memory, can run Vista acceptably. It seems that 1G just isn't enough.
I also "upgraded" my graphics. The PC is a low profile case, which limits dramatically the video it can support. The Radeon 9200, which had been a fine solution under XP, didn't support any of the new Vista graphics features, and some of the system software (like Windows MovieMaker) wouldn't work with it. In fact, it seems that any certified for Vista software for video editing will likely run into this problem because of the changes in the Vista video subsystems. So, I went looking for an AGP low profile graphics card supported by Windows Vista — a hellish problem, it turns out, because most manufacturers have moved to PCI-Express connections. Eventually, I bought a Radeon 9250 on EBay (price $40) which solved the problem. Not state of the art, but Vista capable.
And now I wait as the inevitable upgrades trickle out from manufacturers, who are finally delivering Vista certified products. Generally, now that the unfamiliarity has worn off, I prefer Vista to XP. The UI is well thought out, the security model is better, the graphics beautiful, and there is much new functionality under the hood. I love the fact, for instance, that it natively supports Nikon Raw format photographs (NEF) from my Nikon SLR.
XP is getting long in the tooth. It might be the best choice for the short term if you don't have the time or patience to spend on debugging an upgrade. But if you can spend the time and a little money, Vista is fabulous.