I guess it sticks differently to different skin, but I also think it has to do with the area where you put it. I try not to put it anywhere where it might bend or crease, and mine has never fallen off (and I've swam a lot), never come off in the shower, and the edges have stayed on.
I had asked my doctor about what happens if it comes off a little. Even if it comes of a little (the edge) it still protects against pregnancy: you still have a significant amount of estrogen in your body (60% more than the pill). If it falls off completely, you need to replace it within 7 days or else, after 7 days, you could become pregnant.
It also really helped lighten my period. I used to lose so much blood that I would feel weak, and the cramps would put me in bed for days, but now I am fine =)
I am a big fan of the patch, but to each their own =)