In reply to UKC Gear: I bought one of these in august, slept on it about 10 times in the lakes. It is a fantastic bit of kit, warm enough to allow me to carry my 3 season bag into December, really comfy (I've never been able to sleep on my side on a mat before) and lighter than my old full length thin thermarest (cant quite remember the model).
My only grumble is it is quite narrow (or I'm quite wide!) so if its too warm to be inside you bag your elbows are on the floor, and as it's 5 cm thick its fells a bit odd. However, it's not been particularly warm this year, so not often a problem!
I strongly recommend shelling out the extra cash on one of these.