I have used 99designs for two logos and used Logoworks back in 2007 for one logo. 99designs produces many more creative and professional designs than Logoworks. Logoworks owned by HP was more expensive and we were ultimately unsatisfied with the experience after being beaten down going back and forth on the little choices we had, even after they offered to throw in a few more "staff" designers.

Getting close to 100 designs with 99designs even though many were variations still gave us far more alternatives that we were happy with. In fact, it became difficult to choose a winning design.

You have to stay active during the process. Offering suggestions on what you want to see and what you do not like. The designers usually do exactly as you say. After their initial concept they usually just make minor changes. Comments like "like this but can you be more creative with this part" does not usually work.

As a startup, I believe it was both cost effective and the best use of our time. Instead of spending time finding a designer with a portfolio we liked and then going back and forth with revisions with them. Almost every revision we requested on 99designs was done within a few hours. I also feel the anonymous process of 99designs prevents wasting time by trying to be too nice with comments.

I have not worked with a designer, how many choices are you really going to get before it starts to be uncomfortable as you are wasting both the designers time and yours. Five designs? Ten designs? With 99designs you might get 20-40 unique designs. It just seems like better chances for success in a short time.