At first I looked at an adapter from Shure itself, the MPA-3C, but at US$75 on Amazon, it's extremely expensive compared to other solutions. It also doesn't appear to support all the functions that newer inline remotes offer, like volume control. The Monster iSoniTalk adapter came out around the same time and is far less expensive, but it wasn't clear from the product description if it supported more functions than the Shure adapter or not, like VoiceOver or track skipping.
Next, I checked out an adapter from iLuv. While the price is certainly right (six bucks as of this writing), lots of people said this adapter breaks relatively easily. Not only that, but the adapter itself is extremely short, with only a few inches of cord leading from the remote. For an iPod shuffle or nano this probably isn't a big deal since you can wear those anywhere you want, but your iPhone probably lives way down in your pants pocket; a remote that's only a couple inches away from the phone itself isn't much of a solution. This adapter also doesn't have a microphone, making it far less useful.
Belkin offers a similar inline remote adapter for around the same price, but it also lacks a microphone, and its cords appear even shorter than the iLuv adapter. This adapter has very poor reviews on Amazon due to its lack of durability, so while it was a relatively cheap solution, the thought of it breaking after a few months wasn't particularly appealing.
The Griffin SmartTalk adapter offers a better solution than either iLuv or Belkin. Not only does the adapter have a cord long enough to make it useful, it also has a built-in microphone. But this adapter also got poor reviews for durability, and it doesn't have volume controls.
The solution I finally settled on wasn't an inline remote at all, but a Bluetooth adapter from Sony, the DRC-BT30 ($65 as of this writing). This adapter supports all the same functions as Apple's pack-in headphones: play/pause, skip forward/back, fast forward/rewind, volume up/down and VoiceOver activation. It also goes one better: if you hold down the "phone" button on the adapter, your iPhone will automatically redial the last dialed number.
The DRC-BT30 came with its own set of headphones, a bass-heavy set that was better than Apple's pack-ins but not as good as my Shures. These headphones had a relatively short cord -- apparently Sony thinks most people will wear the adapter itself near their heads -- and they also had a behind-the-head "neck chain" design that I've always found uncomfortable. The good news is the Bluetooth adapter can be used with any set of headphones you like.
The DRC-BT30 is rated at around seven hours of battery life (120 hours of standby time) and charges via USB. In my testing thus far, Sony's battery estimates have been pretty close to the mark. Audio playback over Bluetooth will obviously drain your iPhone's battery far faster than through the headphone port, but I was able to go on a four-hour trek the other day with this adapter and lost only about 33 percent charge, even while doing other things like surfing the internet and checking mail. Sound quality for music, movies and games is virtually indistinguishable from what you'd get by plugging your headphones into the iPhone itself, and as far as I can tell, there's no audio lag at all. The adapter will work just as well with a Mac's Bluetooth connection and supports both audio playback and microphone functions, but oddly enough Sony's own PlayStation 3 will only use the adapter as a voice chat headset.
The Bluetooth range leaves something to be desired, though I think this is more the iPhone 4's fault than the adapter's. Bluetooth range with my MacBook Pro was excellent, but audio playback to the iPhone 4 started skipping if the adapter was more than a couple meters away or blocked by more than six inches or so of intervening material. For example, holding the iPhone 4 on one side of my hips and the adapter on the other side caused all kinds of audio dropouts. Apple's discussion forums are alight with complaints over the iPhone 4's Bluetooth audio performance, so it's probably not an issue with Sony's adapter.
Microphone performance is hit-or-miss. I've included some sound samples below, comparing the adapter against microphones in the MacBook Pro, the iPhone 4 and Apple's pack-in headphones. You'll hear that when it works well, the adapter's voice quality compares quite favorably to the other microphones -- but when it works poorly, it sounds really terrible, with all kinds of compression artifacts. I can't tell why the quality occasionally dips down so much, but even at the lower quality it's still intelligible, if somewhat swampy.
Microphone samples of a Mark Twain quote for comparison:
If you've got a pair of headphones you love but that don't have built-in controls, and you're willing to make the tradeoffs in battery life that come with Bluetooth streaming, I'd recommend the DRC-BT30 or a similar adapter over any of the corded inline adapters I've seen. The adapter works quite well in the car, too, making switching between tracks and controlling playback several orders of magnitude less dangerous than using the iPhone itself (seriously, never try to use the iPhone's touchscreen while driving. Only maniacs do that).