You shouldn't drink and drive just as much as you shouldn't smoke and drive. I don't know the statistics, but I don't hear much about people getting high, driving, and crashing/killing people. However, being a mind-altering substance, it's probably not the best idea.
I don't think it's a gateway drug. Once again, I don't know the statistics, but if I wasn't at work, I'd look them up. I know plenty of people who smoke and have never tried anything harder, and have no desire to. I myself fall into that category. I've smoked since high school, and still, have no desire to be a crack head or end up with meth mouth.