As an illegal good, bitcoin would be MUCH safer on a per-user basis than weed. Especially considering the fact that in order to enforce laws against it, they will have to spread themselves even thinner than they are, and that's so thin that they can't even keep heroin out of their own prisons as it is.
There are a ton of factors involved here. The utility of the goods for the user, availability of alternative legal ways to obtain a similar good, the chance of getting caught, the severity of the punishment... Most drug users will probably rate the utility fairly highly. There is no legal way to obtain something similar, the closest thing is probably alcohol. The chance of getting caught is very low, and if you do the punishment isn't all that severe.
Bitcoin users on the other hand could usually about as easily use paypal or credit cards. The chance of getting caught is unknown. However, seeing how RIAA and MPAA are allowed to just collect IPs and send bills for infringement I have no doubt governments could pass laws that make it illegal to connect to the bitcoin network altogether. Then they can just start collecting IPs and punish a collection of users severely, while letting everybody know how easily they found them. This would scare the average person back to paypal and CC, and make it that much easier to find and punish the remaining hardcore Bitcoin who actually has something to hide and can be bothered to use proxies and stuff like that.