I wasn't sure with the concept of contest for a logo at first, but my experience with 99design was just mindblowing.
Some of the logo that were submitted by designer were not exactly following the suggestions I first gave, but there is always possibility to give feedback so that's not such a big issue.
Most of the time, the designs were spot on. It was difficult to make an informed choice! After choising my favorite designer, it was very nice to see that he was gladly altering some little details so that my logo became exactly as I envisaged it.
I really enjoyed my experience with 99design and strongly recommend it.