I'm on my second cashback deal at the moment. First I got 12 months free on a 12 months contract on from e2save. At the moment I'm midway through an 18 months free on an 18 month contract deal from the Link, and have received all the cashback cheques due so far. However, I've always been wary of recommending deals to friends, since I'd feel bad if they didn't work for them.
http://www.reestitmutton.co.uk/ has a big database of cashback deals you can compare, with affiliate and non-affiliate links to the deals. If you're a member of certain cashback websites you can often get 10 or 20 quid depending on the retailer.
A few other thoughts:
Only take out an offer like this if you are certain you can follow the instructions to the letter. Put the claim dates in your diary, alarms on your mobile, etc so you don't forget.
Some of the companies play games - as you've mentioned the claim processes are convoluted, and you might need to spend some time chasing up your claim. You might need to take it to a small claims court, but if you've followed the instructions, you will win. Whether you can be bothered with the hassle is up to you. Personally, all my claims have arrived within a month of when they should have, requiring at most an email asking where my money was.
Depending on the claim instructions, whether you want to move your number to your next contract, you might end up paying for an extra month at the end of your deal. Having said that, the effective monthly cost of, say £30, spread over 12 months isn't that bad.
I think the most important thing is to choose a company that won't go bankrupt before you get your cashback. Your contract with the mobile network and cashback deal with the retailer are separate. If the retailer disappears, the network will shrug their shoulders and expect you to carry on paying the monthly bills.
So phones4u are a good bet, whatever the previous poster said. If they don't pay up, small claims court them. e2save and onestopphoneshop are owned by the carphonewarehouse, so are a good bet in my opinion as well.
Hope that helps. To conclude, don't sign up if you can't afford to lose the money, but if you chose retailer carefully, follow the instructions properly and are prepared to spend some time writing threatening letters, you should see your money.