I'm an asthmatic, so i'll post my personal experences:
I bought a vaporizer a few months ago (an aromazap), and i find that it's benefical for my asthma, however, using more standard smoking devices like joints, pipes, and bongs have only made things worse. Pot is extremely useful for treating asthma (and is without the unpleasant side effects of steroid-based inhailers), but it's not a wonder drug