Keep in mind that 1 device = 1 very overtaxed, non-replaceable battery. If you're just doing routine workout loops over familiar ground, or have a really good memory and only need to consult the phone map occasionally, it's no problem. But, if you're like me and you like long "adventure" rides in unfamiliar areas, you probably want something that will give you a continuous map display for several hours. Continuous GPS nav drains a smartphone battery like crazy. Of course you can add a weatherproof bar mount, external battery, etc. so that might be workable, but also adds complexity. I've never tried it so I don't know how much more battery life you can realistically get.

Lately I've been using a Garmin Oregon 600 which uses rechargeable AA's so I can go indefinitely, but there are other things about that unit that aren't as nice as the Edge series. And, in general Garmin maps and firmware aren't up to the quality of the continuously updated online maps on phones (except for Topo).