Like Tone, we looked at it last year when changed but as the Mrs doesn't do many miles it just didn't seem worth it. Didn't get into the nitty gritty of the small print but have heard horror stories about the things T mentions above and folk having their pants down when they return the car particularly. The mileage restrictions is a big thing so you'd have to really be careful with that as the excess mileage catches folk out too.
There are some cracking deals about at the minute with decent manufacture & dealership contributions but there are also some good discounts being thrown about if your buying new ( allegedly ). Bastard mine field tbh !!!