My current USB devices are:
Keyboard
Mouse
Digital camera
Printer
Scanner
MP3 player
IrDA adapter to sync my Palm
And at least one hub or two somewhere along the way
At one point in time, I had all of these devices (minus one hub) connected to my main PC. Heck, I installed SIX operating systems (WinME, Win2K, WinXP, Linux, FreeBSD, QNX RTP) with said devices connected. Granted, I didn't get all of them working in every OS, but all of the Windows OSes were fine.
The only reason why I don't have all of these deives connected to one system now is beacuse I have set up an XP "server" that allows me to print/scan/play-n-serve-MP3s over my home LAN.
BTW, I plan to go dual Athlon with these components very soon. Should be wickedly fun.
THe key with USB, for me, is hot-plug-ability. I can unplug and replug nearly all of my USB devices without hanging up something.
-SUO