To be honest I'd go as far as to say they're borderline unusable for anything graphical (and I've spent more hours than I care to admit poking around with them!) Overclocking is easy and when pushed to 1Ghz that definitely helps a bit, but nothing out of this world.
The one exception I've heard of is as a media centre - apparently XBMC is very, very well optimised to the Pi's hardware and thus it works pretty well (so I've been told.)
Aside from that, as a low power robotics "brain" it also works very well.