Oh boy. Slicks vs. treads. This is a can of worms. This should be a topic of it's own.

As for the original post, cruisers are great for trail riding. The only problem comes when some guy with multiple speeds goes flying past you. Keep in mind that there is a minor trend toward simpler trail ringing bikes because trying to keep up with all the latest mountain bike stuff is expensive and you are always chasing after something better. The goal is to ride and enjoy. Cruiser do great off road. It can be more of a challenge when you only have one speed and a coaster brakes, but that also makes it interesting.