I'd say I'm an average user. I use WinXP for the Internet and some light word processing and multimedia.

I'd say it's a good upgrade for beginners/intermediate users, as it adds more security and its own popup blocker. Since I already had the Google toolbar, I left it. I also left my firewall, ZoneAlarm. but these things are installed seamlessly in many cases for beginners, so it might be good to have good security right off the bat.

As for expert users, I'd check it out and see how it modifies the OS.

I lost nothing when I upgraded. I used Windows Update to grab it and install it.

T