In reply to Indy:
Heat Pump dryers are indeed a lot more efficient, but are a lot more expensive initially.
If you use a dryer occasionally, a vented dryer is best. It dries quicker and in all ambient conditions. You obviously need to 'vent' the steam somewhere, but lots of people just put the hose through an open window.
Traditional condenser dryers are better than vented, in that they don't need a vent hose. They are generally slower than vented dryers, but more efficient due to the way they reuse hot air which allows the heater to be used less for the same heat output. They can sometimes struggle in high temperature and high humidity conditions.
Heat Pump dryers are a modification of a condenser dryer. They still use a 'heat exchanger' type system, but replace the electrical heating element type heater with what is basically a backwards fridge. Drying times about the same time as a condenser, but seem to handle higher temperatures/humidity better.
Which one to get? For the average family, I reckon a B Energy condenser is the best compromise. Get one that has 'Sensor Programs', as they are more efficient than a guessed timed cycle. To save energy, put more stuff in the drum than you think sensible...