I have a Surly Cross Check and it feels not much different than the road bike that it replaced (Specialized Allez comp - hit by a car). Honestly, I feel like, unless you're racing, you don't necessarily need a road bike. I like 'em, but I like my Cross just as much, and I feel like the speed difference is very minimal (and I even have bigger touring wheels on it).

I have also owned mountain bikes and again unless you're doing DEDICATED mountain riding a lot, you'll probably be fine with a cross. BTW, once, while trining for an AIDS ride I did eighty miles on my MTB with slicks one day (just showing that it's possible).

What you get with a cross bike is a lot of flexibility. How you set it up will obviously determine A LOT. Get some drops and a lighter pair of road wheels and you have a perfectly good road bike with slightly relaxed geometry. Get some bigger wheels and a straight bar and go tackle some pretty crazy trails. If you're handy with a wrench, you can do whatever you want, practically whenever you want. I make mine different depending on the seasons.

I hope this helped, rather than confuse more.