Tix 4 Tonight is a great company if you are flexible about the shows you want to watch and the seats you want to get. To give you an idea of how much of a discount you receive, our party paid $77/person for tickets to Cirque du Soleil's Mystere. We were also considering KA for $97/person.
The earlier you go, the better. I'm talking roughly 8-10AM. When you pay, they give you a voucher for a group of seats in that price range and you have to go to the specific theater's box office(in our case, Treasure Island) to get actual assigned seats. (It's like checking in at the airport to find out what seats you're getting.) Best do that earlier in the day so you can spend the rest of the day not worrying about making it to the theater on time before the show.
Overall, I have no complaints. The attendants have iPads to help you pick a show/attraction when you're in line and tell you exactly what the prices will be when you get up to the booth. They were really friendly and helpful. It's a great way to find a show in Vegas.