I have an X-Trail - great car but there are no AT tyres on earth to fit it, well none in the UK anyhow, and despite Nissan continuously repeating that their surveys show that 30% of owners drive off road once per week (or month maybe) they have no interest at all in trying to source AT tyres that might fit.
I managed to get some of the Yokohama ATS tyres that Tim mentions above, to fit the X-Trail, and must say that I've been impressed. They aren't a hard core mud tyre but as a bonus they are snow rated and do very well on the snow which many AT tyres don't. They are no nosier than the standard fit tyres that came on the car and I would guestimate that the MPG hit is in the order of 1 - 2mpg at most. I've also not noticed any performance hit when driving on road in terms of wet grip or braking distances and, again, that can be a problem with some AT type tyres. In terms of wear I don't think I'm going to get the stellar mileages that some people report for some AT type tyres, I guess such a hard compound just wouldn't give the on road performance, but the tyres have probably 17,000 miles on them now and look good for another 8 - 10,000 on the eyeball test. The will also have a significantly increased resistance to puncture compared with a more road biased tyre. I'd certainly recommend them for moderate off road use in situations where you might do most of your yearly mileage on road - I do around 15,000 miles per year but only about 20 miles per week off road. The snow rating is an added bonus and an area where some other AT tyres fall down.