Truth and opinions are both OK. Its outright lies that aren't OK. You could say, "He was driving in the right lane at 8:25pm while honking his horn and weaving like a monkey on crank." and the only thing that could get you in trouble would be if he wasn't actually driving in the right lane at 8:25pm. The opinion is just that, a matter of opinion, and can't be considered libel.
peace,
sam