Admittedly, an inflatable sleeping mat falls further into the category of camping equipment than travelling equipment, but I always take one on long trips because you just never know where you might end up for the night: bus stations; train stations; hard wooden shelves masquerading as beds; the rough decks of cargo ferries traversing Lake Malawi, among sacks of cement, iron girders and piles of snoring African children. In addition, they're handy to have if you do go hiking along the way, and they're compact enough now that their usefulness outweighs the pack volume they consume.
Therm-a-rest used to be the Kings of sleepage with their patented self-inflation design, but recently Exped have matched T-a-r blow for blow with their flagship Downmat series, like a young Swiss pretender usurping the crown of the ageing colonial curmudgeon, lazy from sitting too long on his laurels. The Downmat has spawned a prolific brood of mat brats, among them the Synmat UL which contains synthetic insulation for warmth yet squashes down to the size of a telephoto lens. This is ideal for whipping out having, say, arrived in Portsmouth after the last connecting bus has left and having nowhere to sleep but a bus stop where drunken locals on their way home from the pub might think it terribly amusing to urinate on you. Or, for instance, having gone to sleep on the deck of a ferry crossing the Aegean Sea between Crete and Santorini and waking up to discover a rivulet of dirty water from a leaky pipe has pooled around your form and soaked completely through your sleeping bag. Oh yes, with a Synmat UL, all sorts of hypothetical (!) situations could be avoided.
The 7 stands for 7cm and the R-value, the unit by which the relative insulation of sleeping mats is measured, is 3.1, which believe me is pretty good. The actual temperature down to which the mat is effective is of course impossible to quantify due to too many other factors. As with any ultralight item my only quibble is the susceptibility of the mat to punctures. Carrying a portable bed is wasted exercise if careless placement results in a hole or tear. Carry a repair kit? Sure, but a pin prick isn't easy to find at 5500m on Aconcagua where bathtubs of water are hard to come by and you're facing further nights on the snow, so cold that your exhalations condense on your beard and instantaneously freeze solid. Still, look after the mat and it's a worthwhile piece of kit to carry.
I can personally attend to the Synmat's usefulness in the environs of dirt-floored rondavels in Zululand. During a solo, four-day hike of South Africa's Wild Coast I spent one wasted day waiting for transport across a wide river, but nobody with any sort of boat turned up that day. I had nothing left to eat so I backtracked by moonlight to a remote Zulu village in search of sustenance. The first child I came across led me to a rotund Mama, to whom I made gestures of hunger. Speaking no English or Afrikaans, she somehow mistook my mime for a request for shelter, and kindly showed me to an empty rondavel (circular grass hut) and left me to bed down on the packed earth floor.
Having failed utterly to procure food, I did what I was told. Several hours later I was awoken very suddenly by the most peculiar sensory combination of cold, sliminess and eye-poking. Ripped from slumber I sat bolt upright clawing at my face while trying to decipher the bizarre sensation to which my sleeping form had been subjected. There was only one explanation that I could find - an errant frog must have leapt onto my face and just as quickly off it in one fluid manoeuvre. I spent several frantic minutes searching the dark hut to locate said amphibian and having found it, fortunately there aren't many corners in which to hide in a circular hut, ejected it. I passed the remainder of the night without fear of being further used as a human lily pad.
The moral of the story is: erm ... get yourself a Synmat 7 UL?