Two headsets, and why I use them.
I have two (one's actually intended for my wife's use in the car), and I'll continue to use them for two reasons:
1. The volume on my iPhone speaker is deplorable, EVEN after adjusting the "85%" setting in the phone's subsystem. Any headset beats the required mashing of my ears to clearly hear a caller while in the car or outside.
2. Convenience and safety. If I'm driving a distance, the headset is on the dash. Why on the dash? Because it takes me three seconds to see the call incoming (headset flashing), press the answer key, and stick it in my ear. I'm floored at how many people gripe about the discomfort while never removing the thing. I never wear a headset unless I'm on a call (with one exception--I'll get to that). Shove a pencil up your nose and see if it bothers you after hours on end; no matter how cushy, it will. Solution? remove the pencil, unless it's serving a purpose there.
The exception: If it's impossible or impractical to remove the headset, I leave it in. Two examples:
1. I wear a headset while on long rides (bicycle). I ride to work, on errands, and for leisure, sometimes for hours at a time. My wife and mother both have sensitive medical conditions, and I never trust the weather enough to keep my phone handy. Phone in saddle bag, headset on head. I answer every call, and IMMEDIATELY establish that I'm in the saddle and, unless it's life-or-death, cannot talk. The last thing I want to see on the road is some jackass screaming over the wind into his headset while he plows into my rear tire, so if I need to talk I'll pull off and finish up.
2. I'll occasionally headset up if I'm getting dirty and expecting a call. Planting in the garden, servicing the bicycle, doing dishes--one press with the back of the hand, and I'm on the call. No soapy / gritty / greasy iPhone.
On to the goods: I believe in thrift. As such, both of the headsets we own were bargains. One moreso, and the quality shows for the price:
Jabra BT2010. It's small, capable, and matches the phone. Noise cancellation is s-so, BUT it has one major advantage: it charges via USB-micro! So, anywhere my phone can be charged, so too can the headset. Work computer? Check. Apple wall wart? Check. Belkin USB iPhone charger for car? Check. Fit's fine, though it took me a couple of tries to realize it's intended for the right ear, meaning a swap to the left leaves the volume buttons inverted. Up is down, and so on. $7 shipped--a price that makes me less nervous when tossing it into my bag for the day.
The Plantronics Explorer 350 is our car-based headset, and it's quite nice. I found it on Amazon for cheap, and do like it. Solid build quality, with good noise cancellation, and nice specs (though some aren't iPhone-friendly until 3.1 3GS). I reviewed it on Amazon, if you're looking for more in-depth. $25 shipped.
Until my iPhone's 50% louder, or I quit driving alltogether, I'll be using a headset.