This is a great service. I don't agree that you need to get there early. We got to a location near the Planet Hollywood around 2pm one day, got same-day great seats for Ka in about 15 minutes. The next day we got same-day great seats for Carrot Top in about 20 minutes. So it's not that bad. The key is to go to the less crowded locations (they have many of them). The one near Buffalo Bill's is good as is the one near the Planet Hollywood; it sounds like the one near Fashion Show Mall is the worst.
The only bad thing is, they don't actually give you a ticket. You get a "voucher" which you have to then take to the ticket box for the show and exchange for your actual seats. This is quite a time-suck in the vast wasteland that is vegas. For instance, to get to Luxor to get our Carrot Top tickets took forever because of all the walking. If you don't mind, though, this is a great deal.