You have to look carefully at the terms of the cashback deal.
I bought through mobileshop with £90 cashback about 18 months ago, on a 12 month contract @ £30/m - good xnet mins and texts bundle. Had to stay on the same tarrif to get the cashback, 1st payment on 5th month, then second on the 11th. The tarrif suited me for the first 5-6 months anyway, but I got a job with a work mobile, and it was common for everyone to use the work phone loads, so I had no need to be on a high tarrif, so I checked the terms and worked it out. I could stay on the tarrif for 7 months x £30 and get £45 cashback, but when I worked out how much a cheaper tarrif was, I got put onto a £14 tarrif forefeiting the cashback, but instead of getting £45 back, I saved £112 in line rental!!!
You have to be really careful and work out if it is worth the hassle, and with most you only have cerain periods to claim in and if you miss it, you won't get the cashback. Id treat it as a bonus rather than a deciding factor to taking out a contract.
D