Yeah they are legit, but you HAVE to read the fine print!
I bought a couple about a year ago without reading all the fine print.
You usually have to buy more than the value of the certificate (sometimes up to double the value!), you can only use it at certain times of the day and on certain days, some restaurants require you be with more than 1 other person, and some that are listed on the site simply don't accept them either! My mom ran into that. She went with my aunt and my sister during the acceptable times listed on the website, and when they got there they refused to accept it.
This is where crunching numbers really pays off. I figured out, because of how it's worded at one place I like to go, that after going through the trouble I save an eyepoping $2 after going trough all the trouble. Honestly I find it's not worth it.