If you don't have hours to spend searching for deals online or don't benefit from student/military/loca/hotel discounts, this is probably is the best option for you.
I saw three shows, Lion King, MJ Cirque and Viva Elvis. I only used Tix4Tonight on Elvis and the Titanic Exhibit, because those were the only ones worth it. Don't believe the people in line trying to tell you what to do. They didn't bother me so much probably because I was by myself, could see through my sunglasses that I was rolling my eyes when they were telling the foreign tourists what to see... and perhaps thought I was local.
Also, check Goldstar first.