I was having a big problem for a few months with spam bots registering user accounts on my blog. Generally, the bots would register accounts assuming (incorrectly) my anti-spam software was more likely to let their comments if they came from a registered account. As you might imagine, this gets pretty annoying pretty quickly.
Well I can say that the last month with it installed, the number of bogus users getting created has dropped to zero- 100% success rate. Anyways, I can’t recommend Register Plus enough, definitely worth checking out if you use WordPress.