As far as I am aware, e2save are a legit company, but that is not to say that they are any good, as I have not used them. There is a thread on
http://forums.moneysavingexpert.com/...play.html?f=14 entitled "e2save problem" with over 1600 posts if you are interested. There are more threads on there about other mobile companies too, so it may be a good place to start.
A lot of the companies offering cashback schemes on mobiles attach fairly restrictive rules in terms of timescales in which to send your bills in to them, and rely on people forgetting to send them in. From what I have read, if you send your bill in too early or too late then you forfeit the rest of your cashback for the remainder of the contract!

This is where they make their money....
However, some companies are better than others. My advice would be to delve deeply into the terms of the cashback offer before you sign up, and just be extremely organised in sending your bills in (by recorded delivery, of course)!