I've used Tix 4 Tonight twice and have saved some money while doing so. Both times was at the Fashion Show Mall location. But the trade off was that I spent a good amount of time waiting in line. The first time I used it was to get Nathan Burton tickets when he was performing at the V Theater at the Miracle Mile Shops. I didn't exactly know how it worked at the time so when I received the discount voucher for the show, I hustled all the the way to Planet Hollywood to make sure that I would get an actual ticket. Fortunately, I was able to use the Monorail to get there halfway. What was cool was that when I got to the ticket booth, they honored the voucher and asked if I wanted to upgrade to VIP admission for $10 more. I took that offer and got the VIP tickets for the General Admission price. Sweet upgrade!
The second time I used it was to buy tickets for Blue Man Group. Once again, waited about 30-40 mins and then bought the best discount seats they had for the night's show. I saved about $20, so the discount wasn't really huge, but that's lunch buffet at Paris so not too shabby. The funny part was that I thought they had given me vouchers again, but they were actual tickets this time. So I ended up running to the Venetian to try and 'redeem' these tickets. That was a waste of energy.
Tix 4 Tonight can give you mixed results, but if you can spare yourself some time and patience, and a little running, you can really end up with some great deals.