While I love the functionality of Apple's microphone-enabled headphones (both the Apple Earphones with Remote and Mic, which comes with the iPhone 3G S, and the iPhone Stereo Headset, which came with previous iPhones), I've found that the extra layers of functionality also bring extra layers of troubleshooting when they're not working right. For simplicity sake, I'll refer to said models as "iPhone headphones" in this post (non-volume control model pictured).
The most common issues I've come across pertain to the microphone. Unlike most non-microphone enabled headphones, which use a standard two-ring TRS connector (tip, ring, sleeve), the iPhone adds an extra ring to support microphone functionality. This is what the hardcore audiophile refers to as TRRS (tip, ring, ring, sleeve).
But if your pockets are consistently filled with lint and dust (or tater tots if you happen to store them there), that extra ring won't make full contact with the iPhone's headphone jack. Which means that you, unfortunately, will lose microphone functionality.
One way to fix this is to remove any dust or lint trappings in the phone's jack. I've found that wrapping a toothpick with a thin layer of toilet paper and brushing it lightly against the walls of the headphone jack helps out a great deal. It's also important to not dip said toothpick in water or moisture. There's a moisture sensor in the iPhone's headphone jack. When exposed to moisture, it changes color and could potentially void your warranty. Also, the usual disclaimer in do-it-yourself scenarios apply: you're doing this at your own risk, so proceed with caution.
Provided that your iPod/iPhone is still under warranty, Apple will also send out a replacement set of headphones should the ones that came with the unit stop working. And I've found that it's a relatively lenient policy.
I once used my iPhone headphones with my iPod nano during a 10K race. Had you seen the sweat on my shirt after the race, you'd have thought I ran my heart out. Which was certainly the case. But, unfortunately, I also ran my iPhone headphones out. While audio functionality still worked perfectly, the microphone didn't.
The question of getting a free replacement set from Apple weighed on my conscience over the next day. To my right shoulder, there was a haloed angel Sang telling me that it was my fault for running with the iPhone earphones in the first place. To my left shoulder, a red-faced devil Sang told me that it was Apple's fault for making earphones that aren't sweat resistant.
And that's when I remembered reading a purported email from Steve Jobs to a customer who wasn't too happy about paying $300 for the inspection of a water-damaged MacBook Pro:
This is what happens when your MacBook Pro sustains water damage.They are pro machines and they don't like water. It sounds like you're just looking for someone to get mad at other than yourself.
Figuring that sweat typically accompanies running, I chose to listen to the devil on my left shoulder. I received a new pair of headphones on my next visit to the Apple Store
At other times you'll find a non-functional microphone no matter which headphones you use. This happened to me once; and upon inspection, it turned out the iPhone was at fault. The genius at the Apple store reported that the lining in my headphone jack was badly worn out and immediately provided me with a replacement unit.
Aside from the iPhone headphones, I find the non-microphone headphones that come with iPods very reliable for me. I run daily, and they've endured the harshest of conditions and continue to work like a champ.
If you like the clicker and microphone functionality of the iPhone's headphones, but haven't had the best experience with their reliability, fit, or audio fidelity, several adapters -- which let you attach any brand of headphones -- are available in the market. I personally own the Philips SHH1610, which I purchased at Target for $10, but there are a host of manufacturers that offer something similar.Readers, chime in. Tell us your good and not so good experiences with the iPhone headphones.