Ignore this guy. If the bike fits you properly it will be comfortable. I've always ridden road bikes or fixed gears as commuters and I've never had any problems. When I rode a mountian bike with slicks the upright position was inefficient and hurt my back.

A cross bike, with its powerful brakes and wider tires will be a more than capable commuter. If I could afford another bike it would be a cross bike for jsut the purpose you are thinking of. Just make sure that it has fender and rack mounts and you'll be fine.