Do those off-road bike tires with almost no tread really work?
Yes, but whether or not they're right for you depends a lot on where and how you ride. Semi-slicks are among the lightest off-road rubber so they will reduce your bike's weight and immediately improve handling, climbing and acceleration (especially if you buy new lightweight tubes to go with your new tires).
Best of all, you'll feel like it's easier to pedal and you'll probably see an increase in your average speed. This is one of the big reasons racers appreciate these tires. But, it means you need more handling skills in corners and on soft, loose surfaces where the tires don't bite as well as knobbier rubber. Added speed helps on technical terrain because it means you carry more momentum into challenging sections, which is a good thing if you have the skills to clean the obstacles you find there.
These tires aren't for every offroader, though. Because they're so light, there's an increased chance of pinch flats. Also, with such little tread, the tires wear out faster. When you're climbing on slippery surfaces, you've got to be more careful to keep sufficient weight over the rear wheel to prevent tire slippage. And, as we mentioned, the semi-slicks can feel a little squirrelly cornering on loose surfaces.
Our recommendation? Ideally, you'd try a pair before buying them. One way to do this, is to find a friend who's using them and ask if you can test ride his bike. Or, if the bike's the wrong frame size, you might try swapping out the wheels for a quick spin. That would quickly tell you if semi-slicks are right for you.