they are rubbish compared to other appliances but if you feel you really need one, some are better than others. the ones that sense when the laundry is dry and stop the cycle then rather than just running a whole cycle are better. we have one of these (zanussi i think) and it is a B rating which is as good as it gets for tumble dryers. but i try not to use it too often and mostly dry stuff outside if it's dry.
i ahve a really high line that catchs the sun and even the slightest breeze
in the summer clothes are dry within the hour... even on a cold winter day, by the end of the day (ie 4pm ish) clothes are dry enough to spend the night in the airing cupboard and be properly dry in the morning.
tumble dryers all use lots of energy per hour.... some just use evenmore than others