It uses Bluetooth LE instead of ANT+. That means it'll work with iPhone 4S/5 (and whatever other phones with Bluetooth 4.0 capability) without requiring an ANT+ dongle. However, it also means that it doesn't work with ANT+ only devices, which is what the majority of Garmin/Polar devices are.
If you insist on using just the iPhone, I think going Bluetooth is the way to go. Otherwise go ANT+ and spend some money on a Garmin/Polar. You should be able to get them used for cheap. Garmin in particular just came up with the new Edge series, so those with upgradeitis may be looking to sell their current device. I myself use a used Forerunner 305 which I bought for $100 with the heart rate strap.
I have the Wahoo Blue HR and use the Cyclemeter app. It is nice not needing a dongle. iPhone must be 4S or 5, will not work with earlier models. I also got the Wahoo Speed and Cadence sensor that works well. I recommend both.