All my closest friends smoke marijuana, and all have been sucessful in their professional lives to various degrees. My friend Jesse while doing research at Penn St. (which is somewhat rare for an undergrad) solved some theorem that's been around since the 70's and has a full ride almost anywhere he wants to do his graduate work. I wouldn't say marijuana helped him, but it certainly didn't hurt him, its his choice and he shouldn't have to be fearful to make his choice.
I've hired people who I knew smoked pot and it didn't bother me because its no indictment on character. People who don't care to do a good job won't regardless of their choices in life outside of work.
As for my kids, I'll let them make their own choices, honestly I hope that they would only start smoking after highschool because its easy to go overboard at that time like I did. After recieving the traditional school propaganda about drugs and then experimenting with pot on my own I began to distrust eeryting I was told regarding "drugs". A more open and honest dialogue probaly could have prevented alot of mistakes I made, but I don't really have any regrets, just embarrasing stories. I'd like for my kids to avoid the same pitfalls