Well - I will be the lone voice of dissent here. I had a mountain buggy for dd, and while I liked it for many of the reasons given here, there were downsides that were problematic.
Here are my reasons for consideration - a mountain buggy is easy to fold down, but needs some foot action, and both hands - so what do you do with the baby? You can't hold a child and collapse the thing at the same time. I finally ended up collapsing it with dd still strapped in. Second - it is a very sturdy buggy, which means heavy. That's fine if you're simply lifting it into the boot of a car, but not so good if you actually need to carry it around for more than a few steps. Lastly, the single handle means there's really no place to hang shopping bags, nappy bags etc., and the basket underneath isn't big enough to hold much. Again, that's fine if you have another method of carrying stuff - car, backpack etc.
All in all, I think the main question is - will you be using it in a big city? If so, spring for one of the topline Maclarens. They're sturdy, easy to collapse with one hand, easy to carry around as they're lightweight, easy to maneuver, and have handles that stuff can be hung on. If you're main use is country/village/small town, then go for a Mountain Buggy. All very much, IMO of course!
Oh, and also should say that I got my Mountain Buggy 5 years ago, so there may be improvements in the new models that render some of my comments above obsolete.