In reply to ffati:
I bought an Exped down mat UL7 (their lightest one - I think it is around 500g) a couple of years ago. I've used it tons, and would never look back. I've literally camped on sticky out rocks and not felt them and I've camped in minus god knows what in the Himalayas and been warm! Before I always used thermarests which were great but not as warm or comfortable as the Exped DM.
I made the huge mistake of lending it to a friend and when it came back it had 3 punctures (she confessed up later to pumping it up in the field not the tent - duh!) I repaired them with glue after I located them by putting it in the bath (only way I could find them) and since then it has been fine. I'm never lending it out again!
The snozzelpump is a bit of a pain in the arse (not as quick as putting up a thermarest) but once you get used to it it's okay.
As for weight I wanted a lightweight mat and went to look at the thermarest Neo air (the lightest one) but I was so sure it would puncture and also it was really noisy.