I take my cross bike on very steep, technical MTB trails about once a week. Sometimes I do MTB club rides on it just to piss of the guys riding $5000 full suspension bikes
It is more challenging than riding the same trail on my MTB, but I can still clean all but the steepest stuff. The wheels take more of a pounding without the benefit of a big low pressure MTB tire, so I have had to touch them up occasionally. Other than that, I'd say my cross is just as tough as my XC MTB. I wouldn't be doing drop offs on it or anything, but you will be surprsied at what a cross bike can do in the dirt-I know I was. With the right gearing, you can handle just about anything if you pick a clean line.
Now....for the other 90% of the time, when you are on the road.... Look out! My hardtail MTB feels like molasses in january in comparason. Every time I get on it after riding my cross for a few days it makes me laugh at how slow, fat and squishy it is. Night and day. Cross bikes really are fun. Go for it.