I love my Surly Cross Check. It's fast (enough) on the road, will take a rack, and handles gravel roads/grass like a champ. That's really as rough as I want to do (it would also be great on rail trails). Unless you're into super technical stuff, I would say it would be fine for mountain biking. Folks race them over all kinds of wicked stuff that I wouldn't do on a mountain bike even.
I really am attached to drops, so the cross check works for me. I just don't like flat bars...go figure...
I would worry about a double, though. I ride a road triple, and still have 12-34 on the rear for steep gravel roads.
Good luck!