I completly agree with Jacob. There is a self esteem boost, but even with that. Every man probably has some insecurity about his penis size. Even those of us who are well endowed can still have insecurities about our size.
Advantages:
The ability to "Fill" a woman more during sex, that seems to be pretty favorable for them.
Also, some women are also very intrigued and fascinated with a large penis. I've had female friends discover my Magnums when using my bathroom and opening the drawer. It's fun to receive comments/compliments in a friendly way.
Once when a girl saw my dick during a one night stand, she started gazing at it, and then started to talk to it, telling my dick, not me, but talking to it about how big it was, and how she couldn't even fit it in her mouth, etc. That was kind of strange.
Its kinda nice being a white guy with a large dick, because that seems to still impress women when the discover what is in between my legs.
Some women, upon learning that you have a large penis will start gossiping and telling their friends about your size, which can be a good thing and a bad thing. But it's been nice in most cases for me.
When I was a volunteer intern, I used to have a lot of 'bitch' tasks to do, for bosses i didn't always appreciate or like. When they would talk down to me, and I just had to keep quiet, it was nice to know that I had the biggest dick in the room.
Once when I was kissing a girl for the first time, she felt me through my boxers, and later told me that "i was only planning on kissing you, but I could feel you getting hard through your boxers. I had never felt anything that big before, and knew I had to have you in my mouth." Which lead to my first blowjob.
Disadvantages:
Once I was kissing a girl for the first time and getting hot and heavy. I got stiff in my jeans and when she felt me up, she looked at me startled, said "Uh-Uh" and walked out.
Causing a girl pain when you have sex with her is not enjoyable for me.
Having to stop half way during sex because she is in tears is not either.
I've talked to a girl or two, who found out that I wore magnums, and then didn't believe that I had a big dick. She was trying to convince me that just because I wore magnums that didn't mean anything, and without even picking an argument, she had it convinced that white guys who wore magnums are all just average.
I once went commando to the grocery store, wearing basketball shorts. I noticed a pair of high school age girls that always were in the same aisle as me, and then I realized that they were walking and following me to look at the outline my dick created in the fabric. While that was entertaining, it made me realize that having them stare, and having other guys stare at my junk bouncing was sorta weird. So going commando is sometimes inappropriate.
I have to wear all pants pleated. If I try on flat front, I feel like I should be charging admission when i walk around because of the show I'm putting on.