It can take up to a year to heal if you have large nipples, and your nipples will most likely increase in size; so don't just get one pierced. The piercing part isn't painful, but what is painful is that your skin heals to the piercing. So the first few weeks, whenever your nipples get hard, or cold, and they shrink; you'll feel all of the outer-layer of tissue rip off of the metal. That's the part that hurts.
Thanks.... Straight to the point haha
I had my nipples pierced for about two years until the weight of the rings made the holes migrate downwards (ouchy.) I'd suggest you stick with bars for the twice the amount of time it takes you to heal.
Also; be prepared for idiots that don't know shit about piercings, as you can already see.
For me though; having my nipple rings pulled on was less stimulating than having my actual nipples grasped and pulled on. I wasn't sure what I thought of my piercings as far as stimulation went until they were healed, but still, they were just eye-candy to me and people never got that. The only physical pleasure I got from them that I couldn't get without them was feeling them the weight of them bounce.
If you do get pierced, don't ever do less than 12g if you can help it. 10g is ok if you have tiny nipples, but 12g won't be ripped out anywhere near as easily.