I've run a couple of draft essays through it, and it has helped speed up my revision quite a bit.
Just FYI, about 50% of the suggestions are useful. It's particularly good at picking up the passive voice. I find the word suggestions less useful. They work okay on relatively unpolished bits of my writing, but are mostly superfluous on the more polished bits. The reason, I guess, is that in the more polished bits the word choice is already quite careful.
I'm spreading word around on my blog, and on FriendFeed.