I do not like being too warm in bed, so it may seem strange for me to buy a heated underblanket. However, I work shift work, and so heating the house efficiently is awkward. I got my mother one last year, and she extols it virtues, so I decided to get one for myself. Best decision ever.
I use the preheat and also maybe an hour on low. On a cold morning, I use an hour of low before I get up so that the central heating has a chance to kick in. Bliss.
I got the size single and use it on a double bed (so it is lose on the base and not tied down). Apart from the controller being in the middle, I have had no problems with it moving around or creasing. Good size, too, both length and width.
I would highly recommend this product. I would also recommend waiting for a good price - the price on this item fluctuates a lot on Amazon (£60 to £30 for the single), so if you can, then wait until it seems reasonable before snapping it up.