I too stopped wearing a bra (a year and a half ago) after learning how it seems to be related to breast cancer (the info on brafree.org is what convinced me).
I had a long and complicated relationship with bras before that. At 9 my breasts started getting ready to grow, and I was anxious to grow into a woman physically, couldn't wait to get my period, etc. so I asked for some training bras and my mom got them for me even though I really didn't need them. After a while I forgot about them until I got to be about 11 and I really started to get breasts. At this point my mom started making me wear a bra when we went out, which I resisted because they were uncomfortable and I felt like they were unnecessary since my boobs were so small anyway. But I didn't think that much about them until I was around 13 and I realized that I could get beautiful, lacy bras from the real bra section and I started to like them. At the same time though I was experiencing my rebellious teenage years and would sometimes not wear them, which resulted in fights with my mother. My parents seemed to have the idea that not wearing a bra outside was basically an open invitation to every man around. Which bothered me very much, to say the least. When I got to 17, they stopped caring so much whether or not I wore a bra, which resulted in me having much less problem with wearing them. When I moved to a foreign country at 18, I wore them more or less all the time as I felt insecure and I was afraid of attracting attention in any way. This also went with a period when I was on the pill, drinking a lot, and generally being willfully unconscious about my lifestyle. I told myself I was wearing bras because my breasts felt uncomfortable without them since they had grown
Anyway, after a few years I started to become more conscious about my lifestyle in general, and happened across brafree.org, after which I realized that the only reason I was wearing bras was because I was afraid of social disapproval and that it simply wasn't worth sacrificing my health and comfort for. It's perfectly possible to dress nicely and appropriately and not wear a bra, and there was just no reason to go on abusing my breasts anymore. Milkymommi, I feel like that too, it was definitely a part of my own growth process.
So I stopped wearing bras altogether and nobody has ever commented except my mom
who seems to think I do it to get attention
I also generally wear a tank top or spaghetti strap under my shirt, or a sports bra when I exercise. I have found that my style has changed naturally after I stopped bra wearing. I have very few low-cut or tight shirts anymore. I now prefer looser, less revealing clothing or sweaters in the winter, and I feel ironically much less exposed in general and more secure in my appearance. My breasts have ceased to be the focus of my appearance, and in a way less a part of my identity. I feel more self-contained and self-integrated than I ever did wearing a bra - which, after all, are usually designed to emphasize just as much as to hide. I feel honest with myself.