It'd be dang near impossible to be a member of this world and not have my writing influenced by the people that cross my path. But with great power, comes great responsibility, or something like that. There are so many interesting people around that it would be a sin not to take advantage of such a wonderful natural resource. :)
But (there's always a caveat, I know), like any other resource, it has to be utilized with care and forethought. You have to go green! It's easy, and perhaps a smidgen therapeutic, to have a stand-in of a high school rival as comic relief, you have to ask: Is it necessary? Does my story really and truly need that element, or am I just being bad? Not that being bad is, well, bad all the time. It's a fine line to walk; sometimes it doesn't work, sometimes it does. One of my friends relayed an unpleasant run-in she had with a waitress. I ended up trying to write the waitress in a piece as payback (in my defense, the juvenile behavior was when I was an actual juvenile). But the character took on a life of her own and contributed a lot to the story, instead of what I had planned. I'm glad I couldn't pull it off.
There's also an inherent danger of the tool becoming a crutch. Relying solely on outside ideas for drawing up characters can backfire quickly. You get lazy as a writer, and when your friends find out all the stories they inhabit, you might find yourself short of friends. So yeah, lose/lose. So go gathering every once in a while. Have fun with the things you find. Just don't over-harvest.