I'm using SP2 without any significant problems - that is, it compares favourably to Windows 2000 SP4 (which is what I was running previously).
I completely wiped my harddisks clean, and installed XP from scratch, then added SP2 from a CD that came with one or another magazine.
I'm not using the Windows FireWall (I'm using ZoneAlarm Pro instead), I have disabled a whole slew of services (including Automatic Update), I'm not running any Anti-Virus software (except on one separate system partition that I use to login to the office VPN - which was, in fact, the initial reason why I was urged to switch to XP). I'm using the classic view (on the desktop, on folders, and on the Start menu and Task bar as well), and I have turned off all kinds of visual effects (which are just highly distracting anyway, in my opinion).
The system starts up a whole lot faster than Windows 2000 used to do, it's apparently just as stable, and it looks amost exactly as Windows 2000 (except for the icon appearances - which are actually the only sign that I'm running a different Windows version now) - so, I'm not complaining.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.6) Gecko/20050317 Firefox/1.0.2