Putting in my 2 cents into the ring about Windows 7 vs XP.
To be honest, I wasn't looking forward to upgrading to Windows 7, mostly because I had my system all nice and setup the way I wanted, been running for about 2.5 years and wasn't really thrilled about Vista to really make the jump. Then, early August, my main drive decided to fail.
Given Windows 7 has been out for a while now, I decided to rebuild the system and run it. Here is how I feel about it...
One - Given I had the option of going from 32 bit to 64 bit is nice. With Home Premium to Ultimate (For the system that melted down, I went Ultimate, for the laptop that followed next on the meltdown, I stayed with the Home Premium), it allowed me to make up for one of the issues I was slowly running into with my XP system which was memory constraints because of the allowance to do either 32 or 64 bit support. This also meant i could make use of more memory than before.
Two - It was a 'clean' break. Given the primary drive failed, I had no choice but to redo my system. One thing I will note, if you want to upgrade and keep what you have, it may not be the 'best' choice, but if you are forced to rebuild like I did, then it isn't a much of a problem. If you have data you want to keep, make sure it resides on another drive. Any particular programs you have, make sure you have them where you can reinstall for the most part.
The cons I see going to XP to Win 7 will be the following:
1) Some compatibility issues - Check software you have... I used a program called UltraEdit, a nice little text editing program that is handy for programmers. The version I had was old and was installed prior to IE 7 release. That version no longer installs because of some changes and of course, won't work with Win 7 because it was literally an old version and having IE 8 breaking the old installer. This is not to say you will have problems, but definitely check your software for compatibility issues first so you know that you have something you might need to update to work better.
2) Time to unlearn what you learned - Windows 7 has placed things ins places that is not the norm of XP. If you have not been on Vista, you will have to 'unlearn' what you are used to with XP. Such as where folder options are, where the Screen saver is, how you configure your network and the like. There is also no classic mode like in XP, where you can get the interface to have things like the old way. This is why you have to unlearn what you have learned or you will be easily frustrated with it.
3) Be prepared with the right drivers - Now, I have not had problems with my system doing a flat install. I literally installed Windows 7 Ultimate x64 with 8 gigs of ram and what not and had little problem getting things the way I wanted to (Barring my legacy SATA drives that did survive the meltdown to show up the way I like them to). I did have to make sure I had updated drivers to get the proper performance out of my system, so be sure you have them on hand to get the most out of your system.
4) Be prepared for some disappointments - For instance, I have a Dell XPS 720 with a 2.4 Duo Core Processor, BFG nVidia 250GTS Graphics card, 8 Gigs of RAM and a Creative X-Fi Sound Card. Of course, with XP Media Center, wasn't getting the full use of the 8 Gigs (In fact, i was 4 gigs with that setup), but the disappointment was the lack of certain controls, such as in the XP side, the X-Fi card had a THX speaker control adjuster. Under Windows 7, there is no option available. In fact, noticed several of my games acted weird with the sound, then finding out I needed to use a program from Creative called 'Sound Alchemy' to help games now identify properly the sound card under the Windows 7 environment. Little things like that. There will always be a bit of 'fine tuning' from XP to Win 7 so be prepared to spend a fair deal of time tweaking with some things at time.
Now, I have both my machine and laptop using Win 7. Enjoying it both, although did see the laptop crash for no reason twice (Once due to a Bios update for the Laptop, another seemed to hard lock on the screen saver), but also enjoyed a little of the features, such as the ability to have it randomly change the background at set intervals, orienting Tasks on the task bar the way I like them and using the Aero Theme to do previews of the applications running came in handy for getting to certain apps without having to trying to figure it out which one it is and also have the Alt-Tab ability to go to the desktop itself.
I have not seen any problems, so far, with x64 OS running majority of the programs I care about, but again, I am not a crazy power user, so most of my needs are not impacted as greatly as others. I will say that definitely consider a clean install if you can. My laptop came with McAfee and some other junk, and I uninstalled it, but made me waste 3 days to find out I need to use a company uninstaller to truly cleanly uninstall something to allow programs to work the right way on the laptop.