I've used 99D on behalf of about a dozen clients and the cheapest we've ever gotten a great logo for was about $1300.
I'm sure some people have been happy with the lowest price point, but every time a client wanted the money back guarantee they ended up using. Every time I convinced them to opt out and spend a little more they were generally happy.
I paid about the same and was happy with the results.
One more note for people considering 99designs. You can increase your award even after you start the project. Meaning, you can try out the Bronze level design for $299 and hope to get lucky but then raise your prices when you see the generic designs flow in.
When I say generic designs, I mean the designer will take what is essentially clip art and change some curves, colors, and lines to give you a "custom" logo.
Again, that might be good for what you're looking for but at least now you have your expectations set properly.
I'm sure some people have been happy with the lowest price point, but every time a client wanted the money back guarantee they ended up using. Every time I convinced them to opt out and spend a little more they were generally happy.