Mmm lovely thread.
My three cents:
1st cent - It isn't up to us to decide what is sinful and what isn't. The church you subscribe to decides that for you. That's why you belong to that church, because you believe that they can teach you how to live your life properly and get into heaven.
2nd cent - There are as many interpretations of holy scripture as there are people. Ultimately, it's your own moral compass that you follow. Focus on fine tuning your actions to fit the kind of person you want to be. Does smoking weed get in the way of you being the person you want to be? Do you feel like you are living up to your potential as a human being? If everything is great, then keep doing what you're doing. If, on the other hand, there seems to be a huge gap between the person you are and the person you want to be, try to alter some of the factors that might be keeping you down. Some people can be high all day and function perfectly fine, while other people may find themselves becoming depressed.
3rd cent - to paraphrase SouthPark: Pot doesn't ruin your life, it just makes you feel OK about doing nothing. A little bit is fine, but if it gets in the way of living your life, then it isn't all right. You have to decide for yourself what role it will play, sinful or not.