For the past few months, I've been using:
Arkon Bicycle Mount for iPhone
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 thanks 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. HOWEVER, the downside is that the two pieces don't have a locking mechanism. In an accident (or maybe even if you hit a nasty pothole), the two pieces may come apart, causing the iPhone to go flying along with the piece it's attached to.
- 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)