Since everybody else seen to be saying good things about it, it's my turn.
It's a great program, but I don't recommend replacing taskmgr with it. In times when I critically need taskmgr (usually when I'm running 100000000 programs at once to see what'll happen), Process Explorer craps out. Taskmgr, on the other hand, actually manages to start against the odds.
That's about the only problem I have