I have the Garmin HRM that came with my cycling GPS, but the ANT+ adapter for the iPhone (and the Garmin running app) totally sucks. So I picked this up:
[VIDEO]http://www.amazon.com/Wahoo-Fitness-Heart-Monitor-iPhone/dp/B00526NF7W]Amazon.com : Wahoo Fitness Run/Gym Heart Rate Monitor for iPhone 4/4S : Heart Rate Monitors : Electronics[/VIDEO]
And it works perfectly. Syncs right up w/Strava, no issues at all with the actual strap and no extra nonsense apps (which the Garmin adapter requires).
Worth noting, this is the Garmin adapter that I tried first (hoping that since I already had a HRM that worked great with my bike GPS, I could save a few bucks):
[VIDEO]http://www.amazon.com/Garmin-Adapter-iPhone-Monitor-Cadence/dp/B005ZSYEOK/]Amazon.com: Garmin ANT+ Adapter for iPhone (works with Foot Pod, Heart Rate Monitor, and Speed/Cadence Sensor): GPS & Navigation[/VIDEO]
It sucks. Do not buy.