I am so sick of repeating myself on this one, but I'll give a brief overview of my opinion:
The actual physical effects of smoking marijuana and how this relates to your gains, lack of gains, or fat loss is usually a personal problem related to lifestyle choices, not a problem with the substance itself. People speak of increased appetite and so forth. This is a personal choice. If you can keep yourself in check while smoking marijuana the only negative I have ever really come up with is the actual damage of inhaling the smoke, as it contains tar and carcinogens. I have seen several reports with conflicting info, but if you aren't smoking a few joints daily I think you're probably in the clear with that one
I used to smoke marijuana daily for years, but in the past few have dropped usage considerably, mainly because I'm sick of buying it and just don't think about it much anymore, not for some ill effect it has has on me. I may not be your typical case, but I strongly suggest to everyone that they realize personal responsibility is a big factor with marijuana use. If you can use it responsibly and not succumb to overeating or laziness, then the real effects are IMO very negligible to BB'ing. I realize this is not a popular belief however, as it does not villify the substance as so many are in such a hurry to do it seems.
I have made many posts with links to data and so forth, there shoudl be some here to search. As for the test levels dropping, no one has been able to prove that to me yet with a decent study that has applicable results, so don't "take people's word for it".