They are in theory legitimate - it's not a completely fake web site or anything- but their web site is totally glitchy, and the customer service is AWFUL.
The upside is that, if you are really lucky and don't have any problems with the web site or picking up tickets or anything like that, sometimes you can get good seats at a good price.
HOWEVER... the check out process doesn't work half the time, and you don't know if you actually got tickets or not.
Worse than the web site is the customer service; it's run by someone who has a chart that says "no matter what happens, tell the customer everything is their fault and their problem and do nothing to help".
AVOID unless is your only option and you don't mind a 50/50 risk of showing up and not having tickets.
Too bad, because if the customer service was good I would use it a lot more.