My first bike as an adult was a mountain bike. And I'll bet many people would think of getting a mountain bike over say...a road bike when considering a purchase. With its knobby tires and front fork suspension, a mountain bike can be great for hitting potholes, going over large curbs and crossing gritty patches of dirt along your commute.
However, they can be heavier and take more effort to ride on the road because their wider, softer tire with less air pressure flattens more when rolling on the ground. I would pedal several miles on pavement and be out of breathe. I thought, was I really THAT out of shape? I didn't know then that the flattening was "rolling resistance" which kept me from coasting along like I would with less effort on a road bike (which have narrower tires, less contact with the road = less rolling resistance like ice skates on ice). I didn't really need to go fast, I just wanted to ride a few miles without huffing and puffing and breaking into a sweaty mess. That was probably because I was trying to keep up with my husband on his road bike ;-)
If you really like going off-road on the weekends (because that's what it is designed for) then a mountain bike could make a tough commuting bike. My mountain bike and I went on a few fun rides together but it never saw a dirt trail or gravel road. Sadly we parted and I imagine it is somewhere eating up mud on the mountains!!