I think in the final analysis, the question of the compatibility of grass with Buddhism is a personal one. We each know, in our heart of hearts, whether what we are doing is true to our path or not. The answer differs from person to person.

That said, my (limited) understanding of the eightfold path, is that it prohibits intoxication. Buddhist scholars have disagreed as to the meaning of this word and whether it applies to all uses of cannabis (or other entheogens).

I refer those interested in this issue to the book Zig Zag Zen, an excellent collection about Buddhism and entheogens.