I've just researched this very topic myself. I was looking for what I thought was an impossible combination of light weight, compact pack size, durability and something thicker than the standard inch or so provided by conventional sleeping mats which I don't find comfortable enough and seem to exacerbate my long-term lower back pain probs.
I found the American site Backpackgeartest.org particularly helpful providing a range of really detailed user reviews for all things outdoors and lots of comments on the Downmat 7 and the Insulmat Max Thermo. There are several other sites that provide unbiased user reviews too, including Trailspace.com.
Rapidly came to the conclusion that it was the Insulmat for me.
The Exped 7 is rectangular rather than mummy shaped, so a bit more wriggle room there. However, apart form being bleedin' expensive, its significantly heavier (910g inc mandatory inflator stuff sac v 624g plus lightweight stuff sac), it packs fine (19cm x 15cm v the Insulmat's 25 x 13cm ), and some sites' reviewers report the Exped to feel almost too warm when the temperature isn't freezing. Crucially for me, they’re both well thick enough for a good night's sleep - Insulmat's 6.3cm v the Eped's 7cm.
On a less serious note, several users have used their Insulmat for a bit of lilo-type water sports, which it sailed through (pun intended), whilst I wouldn't want to risk an expensive load of feathers blatting down the river on an Exped.
Managed to bag a new Insulmat on eBay from the States for $32.56 plus $13 postage to the UK i.e. about £23. That's a lot better than the cheapest I could find the Downmat 7 in the UK - which seems to sell for around £90.
Not had chance to use it in anger yet, but it seems really comfortable. My one concern before I bought it was that it might be too narrow and easy to slip off it in the night. However, in practice, and with the mat slightly underinflated as recommended, it doesn't feel narrow at all - my weight seems to depress the inner tubes more than the outer tubes, meaning the outer tubes feel that they contain my body quite snugly, if that makes sense.
So all in all I'm well-happy with my Insulmat and looking forward to giving it a proper outing shortly.
On the subject of full length v short versions of sleeping mats, I spoke to a couple of friends who've used half length sleeping mats - they both regret not getting full length - legs getting noticeably colder in winter, much more uncomfortable, condensation issues and even ice on the non-mat sleeping bag under surface in the morning despite sleeping on jackets etc - nobody seemed to have too much good to say for shorties apart from weight savings, but the Insulmat full length mummy is barely any heavier than the Exped 7 half length, so no contest for me in the end.
