I'm not an expert, but for 3 months I rode my mtb on the road. I was putting in 20 miles or more 3 or 4 times a week. I couldn't go as fast as the people with road bikes, but I certainly worked hard. And I felt more secure and more in control (wasn't used to road riding at all). I was doing it all with my knobby tires too. But I put slicks on them and it was great. The slicks allowed me to go along a little more smoothly. I had a really hard time transitioning to my road bike after all that and I am using my mtb now as a commuter because i just feel safer riding it in the cold and wet conditions right now. I personally loved it. I understand now how my road bike is different and in some ways more efficient, but I honestly kind of miss my mtb sometimes.
Good luck and have fun,
Gray