I've had the same issues. The search is dreadful. I was going to open a ticket for the forum module when I got a chance (I think it's the same as this forum on SS). I continuously have to go to Google to search this site.
I also noticed that it didn't seem to search the Titles of the posts http://silverstripe.org/forum-module-forum/show/267343. I've yet to hear anyone tell me otherwise.
It also doesn't seem like there's a way to search specific Forums either.
I agree that forum searches could do with being a bit better. The ability to search specific forums would be a god send, or at the very least, the ability to search for a whole phrase enclosed in inverted commas.