tl;dr: The iPhone microphone I've been waiting for, just a little more expensive than I'd hoped!
[Note: You can win a FREE smartLav, courtesy of Server Check.in (an awesome website monitoring service), by posting a comment on this blog post.]
For a long time, I've carried around an Audio-Technica ATR-3350 lavalier mic and a special adapter (TRS to TRRS) that allows the 3350 to be plugged into my iPhone, and this works pretty well, but doesn't give me the audio level I would like for a radio-quality audio recording, and it's slightly inconvenient, since I need to carry the 3350, the adapter, and some spare batteries. (See my article External Microphones for iPhone 5, 4S, iPad and iPod Touch Audio input for more notes!)
The RØDE smartLav is pretty much exactly what I've been looking for since I first started recording videos and podcasts on the iPhone. It is well-built, matches the input impedance of the iPhone's headset jack (~600 ohms) well, and it fits in the pocket without requiring a little bag to hold any extra adapters/batteries.
The sound is very good—as good as I get with any other mic through the headset jack. Listening to a recording with this mic and a Shure SM58 through a VeriCorder XLR adapter cable yields almost no discernible difference (from 6-8" from my mouth)—there's just a little more background noise with the smartLav. It's much better than the built-in iPhone mic (which is only useful if you're within a couple inches of the iPhone... which is awkward if you're giving a speech and holding the iPhone right by your mouth!).
Build Quality and Physical Characteristics
The cable feels solid, as does the mic capsule and connector. The only downside to the cable is its length; it's only really long enough for recording with the iPhone in your pocket, or, for video, being pretty close to your iPhone. I like how other lavs have longer cables that let me mic talent on-camera 5-10' away, comfortably.
If you want to extend the length of this mic's cable, buy the StarTech 2m TRRS Headset Extension Cable and plug it into your iPhone, then the smartLav into it. It won't degrade the sound quality at all, and it looks nice!
I use FiRe 2 (Field Audio Recorder) for most of my audio recording, and either FilMic Pro or the built in Camera app for video recording. This mic works with all of these apps, as well as Rode's own recording app, equally well (though some apps don't allow you to change the input level). Since it uses the headset jack, you should be able to use the mic with pretty much any audio/video recording app. You don't need any special kind of recording software or hardware to use it.
One note: If you don't want your recording interrupted by a phone call, make sure you switch your phone to airplane mode before you start recording!
Price and Recommendation
Finally, the price. It's a little high, I think, for a microphone that can only really be used with certain smartphones (especially since the headset jack's input has a reduced range, compared to what you can get through the dock/lightning connector). But it's not outlandish, and for the convenience, I'm fine paying $60. If the mic were closer to $40, this would be a no-brainer.
I'd definitely recommend this to anyone who's trying to mic someone with a smartphone for podcasting, interviews, at a wedding (have each of the main participants clip one on and record with the iPhone), etc. I received a free smartLav from RØDE for this review, and if it counts for anything, I bought another one.
Buy the RØDE smartLav on Amazon for $60. (Note: As of 3/28/13, it looks like the smartLav is selling out about every other day, FYI. Just be patient, and it will be restocked!)