Hi,
Had been using smart phones as my bike computers for years until last week when I finally but the bullet and got and edge 800.
Brilliant bit of kit and worth the money for me as I like time have heart rate and cadence and all the training stuff on it. Got the OS maps which are great for road and off road use. I use the zoom to road atlas (you get both road atlas scales in the maps) for road exploring and 1:50000 for anything off road.
The road navigation is very good, just like a Google Maps type thing.
The computer function is very good with up to 10 data field available on the screen.
They are not cheap but are the do it all device.
Using your smart phone is always a bit of a compromise. The last one I had was a Motorola defy which was water proof so could be mounted on the bars easily. Lots of great apps which will do everything the Garmin will do (others than cadence) but with a much better screen. Battery life was okay, about 5 to 6 hours if you use back light settings. Great for Strava too.
Of course you can just get a basic cheap computer for a few quid and but more beer!
Regards