I had one a few years back with Phones4u without problems. If they're from a big provider I'd expect them to be legit, but the conditions and time limits for cashback have to be kept to very strictly and can be very easy to miss.
Use a more recognised company, rather than studentphone, which my laptop refused to open, and you will be fine.
If they're pretty well-known, reliable companies, they're usually legit but you miss a deadline and you're left with nothing.
I'd recommend looking at the tarifs first - go as cheap as you can and if the company goes bust you're not left paying a ridiculous amount of money.
Quidco ones are fine as long as you ensure it tracks correctly.
I got cashback on my contract from mobiles.co.uk. If its a big company I would go for it. Also, perhaps take out a phone contract that you would happily pay without the discount and any cash back you receive is a bonus!
you have to post the contract thing back, we did on recorded delivery and they said we never sent it!
they owe us over 200 pounds in cashback money (but this was 3 years ago-ish)