I've had five phones from e2save now. All of them have been completely free - i.e. get all the line rental back by chequeback. Currently got two phones on O2 £45 month tariff. When I get to month 11, I give notice on the account, look for a new deal and then port the number.
OK, you don't get the latest sexy phone this way, but I haven't paid line rental on a contract phone now for 4 years.
They are part of Carphone Warehouse so have a big company behind them and not likely to go bust owing you your cashback.
All you need to remember is you must follow the claim back procedure to the letter - i.e. 28 days to claim, must use the proper claim form etc. I guess they are reliant upon people forgetting - but just set a reminder for the whole year in Outlook or similar and it will soon remind you.