As others have said, depends on what is being smoked and what strength though it's very distinctive. Some people seem to be able to smell it better than others though, I pick the smell up really quickly when others say they can't smell anything?!?
It can be sickly sweet with a bit of a kick to it. I found it overbearing walking round Amsterdam by the cafes. In my housesharing days loads of people used to smoke it, worse was waking up every morning to the stink of really strong stuff hitting me after one of them smoking in the night in a basement flat with the windows closed.
You can smell it a lot round my way, especially in certain parts of town and some with smoke it on buses - in the winter with all the windows shut, people seem to scared to tell people to stop smoking it so most of the bus probably ends up passively stoned - can't say my arthritis complains though :)