I really love mountain bikes, how about you?
I have a friend that does it - they are a bit different from regular bikes as they are enhanced for durability and performance in rough terrain.
That is true, they have special wheels that allow the bikes to perform better on and off trail.  Biking on the beach is off trail too, and that is hard!
She rides hers mostly on mountain trails, fire roads and other unpaved surfaces.
That's great.  It's great exercise.
I didn't realize they were popular with couriers who have to navigate potholes and over curbs also.
I did not either!  Come to think of it, I have seen couriers with some sturdy bikes.  They are mountain bikes for sure.  I actually had a ten speed and was hoping to have a tire that was more like a mountain bike - because of the pot holes!  I could only get normal tires for it.
There have been a lot of subtypes of mountain biking, such as cross country, downhill, track and slalom - so it's pretty popular
It is popular, more popular than ever.  If only there were more people that could fix bikes!  I have had to look long and hard to find a good bike shop.  Once you find the right person, like the right mechanic, it's like gold.
There is a skilled mechanic named Brian Phillips who specifically works on Roberts cycles for maintenance, but that's the only one I know of.
That's great.  Thanks for chatting today, you know a lot about bikes!