Heart rate monitor for iPhone

I use endomondo on my phone to track my distance and speed, but thinking of getting a heart rate monitor - mainly to get an idea of whether I'm pushing myself hard enough or should be trying harder! I'm only doing 5k, but aiming to increase to 10k and thinking a heart rate monitor might be useful in determining how rapidly to increase my efforts. Can anyone recommend a decent one that will work with an iPhone? Or is this a silly reason to use one? And do they tell you what your HR is as you're running or only at the end or can you set it to alarm if it gets to a certain level or something like that?

I suspect this is going to be a growing market, as I've seen a few similar requests.  The problem at the moment is that most fitness gear is ANT+ whilst phones etc use bluetooth.

Wahoo seem to be one of the leaders in getting the two protocols to talk and sell a popular ANT+ adapter that would allow you to connect ANT+ heart rate straps to your phone (and upload the data to various apps).  They also make a bluetooth heart rate strap called the Wahoo Fitness Blue HR (available in the Apple store, and I suspect elsewhere).

Have a look on the DCRAINMAKER web site as I'm sure he did an in-depth review of the ANT+ dongle, and he may have also covered the HR strap.

Thanks, that website's very helpful. I'd have thought it was a 'thing' that people commonly used, but I guess not.