Two to four months to heal for a guy or two to six months to heal for a girl for nipple piercings.
The worst part is the clamp. Imagine an pissed off girl biting on your nipple and not letting go. That's what it feels like (or so I've been told). The first few hours after a piercing, it feels kinda sore and weird. Feels like a piece of metal hanging from your nipple. After about 4 days, it feels fine though. You won't even know that it's there unless you think about it.
The positive?
New shiny hardware hangin' from the nipple. You do it for yourself, not for someone else so who cares what others think?
More sensitive.
The negative?
No oral contact to the nipple until it's fully healed. Apparently the mouth contains the most bacteria and is the dirtiest part of the body. Who knew?
Gotta cover it up with a waterproof bandage when going swimming so you don't get it infected.
Aftercare is pretty simple. Spray some saltwater on the piercing 4 to 6 times a day. I've heard H2Ocean is great stuff.
www.h2ocean.com
Worst that can happen is it gets infected. But as long as you take the appropriate aftercare steps and keep it clean, you should be fine.
Remember though, once you pierce the nipple, it gets bigger and it'll stay that way even if you take it out. However, it does become more sensitive (or so I've been told). Not as drastic as getting a tattoo but your nipple will look different when you get it, that's for sure. So if you want your nipples to match, get em both done.