I ran Vista since day one, on my relatively old desktop computer. Vista actually ran fairly well, and for the first year I had few performance complaints. Stability was good too. In the entire time I had Vista, I recall only one blue screen of death. That was due to an old video driver, I believe, and it never happened again.
I dutifully installed SP1, and Vista seemed to work fine. But then, over a period of many weeks, performance began to suffer. Each day, Vista slowed down a bit. I tried everything I could think of, for instance I ran CCleaner every night, uninstalled most apps, disabled the sidebar, disabled indexing, defragged routinely, etc. I even manually tweaked the page file settings.
Nothing helped, and no particular process in task manager was an obvious culprit. Vista SP1 just kept getting slower and slower, until it finally became unbearable.
Poor Vista performance is another reason I recently purchased my MacBook. I had the option of buying a high-end PC, but I had little confidence Vista would use that power efficiently. I hate spending a lot of money on computer hardware, but I hate waiting for sluggish computers even more.