A few more things:
1) I would like to know that it can hold my phone in the event of a crash, and not launch it skidding across the road, possibly into the path of traffic. This is one of the downsides of using a phone as a bike computer.
2) The weather is another reason I would hesitate to use my phone as a bike computer. I'm fairly sure my Garmin Edge 500 is more waterproof than my Nexus 4, although this wouldn't be a concern for everybody.
3) Once you're a more "serious" cyclist and have heart rate monitors, cadence sensors and power meters, you will probably have connectivity issues today. Most of these devices today use the ANT+ protocol, but very few phones support that. Bluetooth SMART is gaining popularity amongst phones, but few power meters etc. support it. Give it a few years and I suspect this won't be a problem anymore.
I suspect phones will become a lot more commonly used as bike computers amongst serious cyclists in the coming years, but there are some very real downsides at the moment compared with dedicated devices. Neat mount though.