Selection: I selected the Thermarest Xtherm for a backpacking trip to Scotland because I was intending to camp quite high on occasion and it can get quite cold, even in May. I normally go for the lighter Xlite model which I think is excellent and ideal for 3 season use, but taking the Xtherm gave me a chance to test it.
When you compare Xtherm to the Xlite the additional weight is very small (just 100grams for a Regular) and the packed size is virtually identical. With the Xtherm though you get a mat with almost double the thermal insulation value of the Xlite and identical levels of comfort. It doesn't 'rustle' as much as the Xlite either, principally because it has a higher denier fabric on the base which also increases durability.
Performance: Some people think blowing up an airbed at the end of a long day is a bit of a chore, and it does take quite a few breaths to fill the mat however it only takes a minute or so and I don't mind doing it. The Xtherm comes with a 'pumpsack' - that is, the stuffsack is oversized and you can attach it to the valve of the mat and use it to force air into the mat and blow the mat up. This operation is less tiring than using your mouth but its fiddly because the pumpsack isn't very big and I lost patience with it and used my mouth. Pumpsacks by other manufacturers tend to be bigger and work quite well.
Using the mat was a complete pleasure, it isn't noisy, its very warm (this is of course anecdotal - but I did have six good nights sleep), and its very comfortable - especially when you compare it to self inflating mats or closed cell foam. In the morning you just undo the valve whilst sitting on the mat and it deflates. Packing the mat away can be done quite easily, even in the confines of a one person tent. First step is to roll the mat towards the valve expelling the air as you go - when you get to the top close the valve. Unfold the mat again and fold into three lengthwise, roll the mat again, tightly, open the valve again as you reach it to let the last of the air out then stow it in its stuffsack.
Other Comments: For most trips I'd still reach for the lower weight Xlite, but if you don't like the way it rustles, or you want one mat for all seasons then the Xtherm would be the best choice. With the higher denier fabric on the base it's also going to be slightly more durable.