Nipples -- No Problem for ME
Hi, I've had my nipples pierced for coming up on 9 years now, and I haven't had any problems with them. Sure, they took a while to heal, but less long than my tongue or my nose. I've pierced a number of things (lots of ear-parts, nose, tongue, bellybutton) and the nipples were NOT the most painful (the 2 nose rings were -- never do 2 at the same time right next to each other).
One thing I do recommend is to switch to bar-bells after yours have completely healed up. The hoops facilitate healing b/c you can rotate them, and you need to be sure to move the bar-bells around if you get them, but the hoops can snag on things. One time, the titanium hoop stripped open before my flesh tore out, which is a good thing and tells you how strong flesh is -- but it wasn't an intentional snag and was a bit difficult to explain at work why my blouse was suddenly bloody (blushes)!
As far as remaining erect, my nipples don't behave any differently now than they used to, and I don't recall any loss of sensation. Just plenty of gain :) Good luck with your decision, and feel free to write if I can be of more help.