I think, as with quite a few tech-related debates, the answer is that it depends what you're using it for.
My dad has used XP for ages (same installation) and it's been absolutely fine. But he uses Firefox, Word, Skype, and that is about it. He religiously does antivirus and Windows updates, uninstalls unneeded software etc.
M.Mouse is right - 7 has similar workings to Vista. However, the core of Vista is actually not bad, and I used / administered it for a year in a corporate environment. Its main problems is that it is extremely disk I/O heavy which kills slow laptop drives, much more RAM intensive (7 is almost as bad but people are used to it now, and prices have caught up), and its networking stack was completely written and thus immature. But, inherently, it's not all that bad, and if you have a decent machine it is probably more reliable than XP, if more annoying and possibly slower to use. Graphically, it is horrible (the default blue green background makes me feel seasick). But it has a bunch of improvements that are actually excellent and very useful.
For the average user I believe that XP is a minefield. I have to use it sometimes and I (somewhat experienced) have to be very careful not to put a foot wrong and break it. If you are doing anything more than very basic stuff with it, or if you are the type to randomly install cool stuff, and if you are not careful / very experienced, you WILL have problems with it. Also, security updates stop in a couple of years which will be fun for you.
7 is Vista with pretty much all the bugbears fixed, and I have had basically zero problems with it with extensive, heavy usage. My computer at work that runs 20 complicated, resource-intensive and buggy things simultaneously all day every day, only has to be restarted once a month for security updates. Excellent OS.