How it will strike you will probably depend on what kind of riding you normally do.
I'd always been primarily a road biker, so the offroading threw me off the first time out, but I never ran out of legs or lungs. I talked to a dude there who was primarily a singletrack mountain biker, and he found the offroad to be lightweight but had a hard time keeping up on the straights.
But as mentioned, every course is different - some have more pavement, some less. Pre-running the course will help you a lot!
I've done only a few races, but in every one I've been in, there are a lot of crashes at the first transition from onroad to off. You can make time there (and in a lot of other places) by thinking outside of the box and taking alternate lines. Ruts form quickly.
I found a delightful synchrony in cyclocross, offroad, and supermoto. The lessons I learned in any one of the above helped me immensely in all three.