Today was the first time I have visited SeatGeek and while I haven't used it to make a purchase yet I certainly made sure to add it to my bookmarks.
Great service that serves a real need (Craigslist/StubHub/etc are all a royal pain to navigate and utilize).
The site has a sophisticated, clean design, one which seems to assume that its visitors will plunge in and begin searching for seats with little hand-holding.
