The use of no substance is inherently sinful, rather it's the state of mind that it produces in the user. If one is engaged in unrighteousness and rebellion against God's glory, then if he smokes marijuana, that will only serve to amplify the feeling of rebellion.
Some drugs are so intensely pleasurable to the flesh that they will suspend one's normal inhibitions against sinning, such as "it's wrong to rob people or steal their stuff," in order to experience the pleasure of the drug again. And many drug users neglect their familial obligations just to repeat the experience of the drug, just to be alone with the substance that they want to use.
But there are some folks, some, believe it or not, who are able to use certain drugs in moderation and not let them take over their lives. The number of people who are able to do this decreases exponentially with the addictive strength of the drug in question. For example, a crack cocaine user has basically zero chance of attaining glorification while using crack. But a marijuana smoker might have a better chance, if he doesn't engage in sinful activities.
This is all with an important caveat: That one must first acknowledge that he is a sinner and has fallen short of the glory of God, and is in desperate need of Christ's saving grace.
There is also another wrinkle to the problem. Certain drugs cause physical dependence. Like for example heroin and the opiates. God undoubtedly created the opium poppy, from which morphine and all the related drugs come, but he did not intend for it to be used as a 24/7 escape from reality. But some people find themselves trapped in that addictive cycle, after having developed a physical dependence on the drugs, because their flesh nature has deluded them into thinking that they are doing something good ("it must be good if it feels so good and helps me to interact with my fellow men on such a mellow plane"). So, would God condemn someone for being sick with diabetes or asthma? No. Definitely not. The same rules for salvation apply to drug users as everyone else. One must accept Christ's sacrifice for their sins and believe on Him in order to be saved.
If one continues to use drugs after he has accepted Christ's sacrifice for him, then it's not a "salvation-breaker," because it's the nature of the behavior that the drug use causes that's the real issue, not the drug use itself.
And I'm going to tie this in with a biblical citation. Where Paul admonishes against drunkenness and sexual immorality (and other particularly grievous sins) in I Cor. 5:11.
This would seem to suggest that just being drunk in and of itself is not a sin per *se*, but rather what one does or thinks while under the influence of alcohol (or any other drug, such as possibly opium - which were the only drugs known to first century people in the Holy Land).
That's just my personal experience.