I have already recommend this site to a few people. I thought it would be difficult finding a designer that would not rip me off in this economy. I knew what I was willing to pay and got so much more with 99designs. The designers were all very nice and professional and I enjoyed working with them.
It was fast, easy, and actually turned what I thought would be a stressful endeavor into a fun project. By making it a contest, and being able to pick and choose what designs I like, I felt like I was in way more control of the outcome. Why work with a design company that will put maybe 1or 2 people on your project, when you can have 10-50 designers working for you but you only have to pay 1 of them?
Number 1 tip I can say is leave feedback on the public page for all the designers to see. Call out the designers that are heading in the right direction so the others know what you're looking for. Then check the site the next day and you will see that all the designers suddenly spit out a bunch of new artwork for you to review. Putting you that much closer to you prefect logo for you.