I know with these things there's always more bad comments than good, but it does seem like you have to be v careful with them, and make sure you check/uncheck the right boxes to not get billed for things you don't want
However I have read that if you don't use your phone that much, apart from the odd call, even if you do pay the 1.99 a month for "premium voice" it's still good value.
BUT, my phone is too old/cheap to run their apps, so by the time I add the cost of new handset, i may be better off getting a cheap virgin contract. (I'm on EE payg)