Tried my first joint when I was 13.
Between 14 and 17 I was fanatically anti-smoking.
Started smoking (cigs) when I was 18.
Smoked a great deal of marijuana from 19 years old, up until about a year ago. Never saw it affect my brain - I've always had an IQ of around 150-160, even when taking IQ tests when very stoned, strangely enough. I did that every three months or so, just to check, but like I say it didn't affect the scores one bit.
I would agree with what Sean Lindstrom said - I believe it actively aided my ability to think creatively and 'outside the box'. It's definitely given me much more perspective about life.
Haven't bought any for months and months, but I will have some perhaps once a week but not more. Last Saturday for example. But I'm aiming to give it all up (cigs and pot) for New Years Eve.
It's not a gateway drug in my case - I've never been tempted to smoke crack or take ecstasy for example (OK, I had an ecstasy pill about five years ago, but it's just not my thing at all).
I find it most enjoyable when I smoke it in places of outstanding natural beauty - some of the most memorable experiences in my life have been: smoking Purple Haze at Castlerigg Stone Circle in the Lake District; Maui Wowie on Haleakala; Maldivean Boatmen's Homegrown at Furana Fushi, whilst fishing on the banks of the St. Lawrence River, and so on. That's almost like a form of religious worship in my eyes. Communion with nature, if you will.
BUT, whereas I used to enjoy smoking it while playing e.g. Civ, now I find I don't like to smoke whilst playing at all. Just seems kind of sad behaviour and gives me very negative feelings. Nice when I'm watching a movie though. It certainly enhanced The Hulk (last Saturday night).
As I say, I'm aiming to give it all up on New Years Eve. I think my body is telling me that too, I certainly wouldn't recommend it to anyone either, it affects everyone differently. It can even give certain people a psychosis, and many will find that it will turn them into anti-social saddo loner types - it's definitely not for everyone!
In general I view it as a thing that has given me a lot of pleasure - but will shortly become part of my past.