Marijuana has pretty much the same negative effects that cigarette has, plus a few extras. There is nicotine in marijuana, and tar, and carbon monoxide. Saying cigarette is bad and marijuana is good is somewhat hypocritical, or ignorant. Add to that that marijuana dulls your mind and make your senses less acute (a bit like alcohol), and it's definately not something that I would call a good thing. Especially if you're driving. Note also, that you can get in serious trouble (in Canada too) if caught driving intoxicated (by marijuana or alcohol), and there ARE tests that can determine the amount of THC in your blood.
Marijuana can have some good in some medical cases (it's a good analgesic for patients in extreme pain, as said before it dulls your senses), but otherwise for healthy people it's a bad idea.