I don't know if you're going to buy what I'm going to tell you right now, but I'll tell you what I've found out, which is that there's no question.... Smoking anything exposes you to carcinogens. The evidence of a relationship between marijuana use and cancer is pretty weak so far, and it's not entirely clear why that is. One plausible explanation is that people tend not to smoke it in anywhere near the volume that they smoke tobacco—depending on the person in question—so the likelihood of its leading to cancer seems to be much lower.
We also have to remind ourselves that research in this area probably hasn't been as extensive as it might have been, because it was an illegal substance. It's something we have to keep a really close eye on, but it certainly isn't a carcinogen that is among our top concerns.