I just got a Cleveland Launcher 460Ti Offset (a Christmas present to myself!). I hit my first bucket with it yesterday.
Now, let me start by saying that I've never been a long ball hitter. My previous driver, a Nike Ignite, would hit a good drive about 200 yards. That is, after compensating for my slice by aiming at the left side of the fairway and hoping it would stay in the right side of the fairway.
Yesterday my game changed. After warming up with about 15 iron shots, I hit my offset Cleveland for the first time and fell in love with it. It did not completely eliminate my slice, which I didn't expect it to do, but it turned it into a very controlable fade. On top of that, It has added about 25-30 yards to my drive.
So to answer your question, an offset clubhead does not take away from distance. In my case, it added distance. If you are fighting a slice, I would highly recomend trying an offset driver.