For the past few months, I've been using:

[Arkon Bicycle Mount for iPhone][1]

I'm very happy with it for the following reasons:

 - Inexpensive (around $13)
 - Very sturdy hard plastic attaches to handle bar
 - Designed to fit different handle bar widths (tighten screws)
 - Can view phone at any angle thatnks to Ball and Socket design
 - Two part design means I can reuse the handle portion of old Arkon Mounts - takes a minute or two to loosen the knob holding the iPhone piece, pull it out, attach to other bike, and tighten the knob.
 - The iPhone portion is specifically made to fit the iPhone 4 or 4s, so it is a snug fit and is easy to insert and remove

That being said - you won't be able to use this model if your handlebar is completely curved. You need at least a 1.25 inch portion of the handlebar that is straight.

I've used this for GPS (Runkeeper), for glancing at notifications, and occasionally for taking a phone call. Busy roads are too noisy for phone calls but on quiet suburban roads I have no problem on speaker phone.

Make sure to get something specific to the iPhone 4/4s such as this model.

(Arkon also makes universal mounts or mounts which work for many models but the model I had like that for my Blackberry a couple years ago required fiddling to get working properly - I mounted extra bike helmet padding to make it more snug)

  [1]: http://www.amazon.com/Arkon-Bicycle-Mount-iPhone-Black/dp/B002CQTYDY