It's the best one I've seen. There is also http://www.instapage.com/ but it's not as good and I've run into problems with pages not loading -- and when you're paying for traffic to hit a page, you want it to load.
The quality of the people coming to the site, in my experience has been the same.
I actually have better CTR on adCenter compared to AdWords for the same ads on the same keywords (marginally better).
So it's really come down to cost for me. In my experiences it's consistently double or more for the top keywords in the bunch.
I've personally never run a massive campaign to drive tens of thousands of visitors to a site through either service. My guess is, if you need high volume of people quickly, you need AdWords.