This works great for the 4S and is compatible with plenty of apps. You can also use it with some app running on the iPad 3 gen which is great if you are exercising at home and want a nice big display showing your heart rate (I use walkmeter 7 for this)
However my unit just stopped working while out on a bike ride and even though I have changed the batteries I can't connect to any of my devices. The problem is there is no indication at all on the strap if it is working at all, so you are left trying to get the apps (and the Wahoo utility) to try an connect.
Hopefully it will be replaced quickly by the supplier (in my case Amazon)