I've used 99designs twice and have had great results. I received several designs within 2 days of posting, and was able to have a final result very quickly. Getting a variety of designs has led to higher-quality, more creative results. I think the price is very reasonable given the ease of use, output, and comparable time and expenses of hiring a designer.
Product : I appreciated having a variety of designs to choose from. My winning designer iterated several times based on my feedback before coming back with a design that was just right. I appreciated the work all the designers put in.