I was wondering if anyone has tried using these before, I periodically check the "sneak peak" section of the Tix4tonight website that lists some of the restaurants that they will be offering 25-50% off entres. Since I am going with a large group, I was wondering if it is actually 50% off or actualy 2 for 1 and thats all you can use.

I've used them twice, and I remember I had to pay $2-4 for the coupon, and then you just present the coupon to the server. There was no issues with it. And it ended up being 50% off food. not on alcohol.
I would just buy it the day of, if you are sure you're going there.
Those vegas flyer books that you can get for free, they have coupons in there for tix4tonight, so you can save a bit more as well.

tix4tonight is legit.
There's no gimmick, it's usually what it says. If it says 25% off entres, it means when they give you your bill, there'll be 25% docked from the entre prices before taxes. Beverages are usually never included in the coupons.

I've used them before and highly recommend it.
It works best if you have a list of restaurants that you want to try and compare it with tix4tonight's list of coupons offered. Make a reservation, and then buy the coupon and show it to your server at the restaurant.

Sometimes the server might forget to add the coupon to your bill, so keep an eye out when you receive your check.