I swapped from RAC to Green Flag Mayday at the start of this year, for the reasons outlined by Mr Grumpy in #4. I've never had to use the service so can't comment on that, but I can say that since we swapped the antique Jeep for Mrs Gozza's new Citroen, GF-M made it quite difficult to swap cars on the policy. They present you with an online form which looks easy enough, but submitting the form just brings an email with a phone number to ring - which you have to try a number of times if you're not happy to be number 89 in the queue.
However, that's a once-every-three-years thing, compared to the peace of mind that comes from knowing you've got specialist caravan recovery if you need it...