I installed it and used it a good deal for several days. Then my system crashed. I lost a LOT of 'stuff'.
Being a glutton for punishment, I rebuilt my system from scratch, reinstalled StumbleUpon and went on my merry way.
Several days later I had another system crash. Wiped out everything, and I had to reinstall Windows and start over again.

My theory:
StumbleUpon itself is not the problem, but the fact that it encourages you to poke around in multiple sites that you would never otherwise visit. This kind of careless browsing will eventually take you to contaminated sites that will corrupt your system.

!!! USE AT YOUR OWN RISK !!! My advise is to STAY AWAY!