I've got a 2004 Xterra. I've only had it for about 4 months, but so far so good... I traded a Subaru WRX for it, so getting used to the truck-like ride and size took a little time, but I'm happy.
As far as the mechanics' comments, I can't really say anything to that because I haven't had it long enough for anything to really go wrong, although I've always heard good things about Nissans in general. The stadium seating in the back is pretty cool although not really necessary. Cargo room is good, the rear seats fold down if needed. I've got the base model so the acceleration leaves a bit to be desired, but it gets the job done. Mileage depends a lot on how you drive it, but I typically get around 260-280 miles per tank (which usually takes about 15-16 gallons to fill when the light comes on, so do the math to find MPG), and that's with mostly city driving. Overall, I like the X.