I know somebody whos tried those, but didn't rate them too highly..IIRC, they were quite nosiy even in a highseat and as Dave says not particularly warm.
The self inflating cushion mentioned earlier make a huge difference to staying warm...I use one by Gelert which is about A4 size and cost about £6, so its not the end of the world if you loose it. Thermarest also do one that a bit bigger but its x3 the price as well...