I've been using a Sony Xperia Z Ultra for Audaxing for the last 18 months.
Its a large phone with a 6.4 inch screen. Sony do smaller phones which have the same properties.
The phone is water proof.
I have a relatively lightweight mount which sits on my handlebars. The phone clips in and out of the mount in a couple of seconds. The mount itself is quick release so can easily be removed or moved between bikes. The phone has never become detached from the holder by accident.
The phone battery will last for 4-5 hours when following a gpx route. This is because of the large screen. A 10,000 mah external battery pack easily allows a 200 km Audax with upwards of 50% charge left in the phone.
I use the ipbike app for route following (downloadable maps) and sensors. I also use osmand for voice instructions if I want them. The two apps can run simultaneously with very little impact on battery life. ipbike can also do the training bit if you want it.
Recently I've also been using GPS tracking, again with little impact on battery life, so Mrs Freeflow can see where I am, and more importantly, if I'm moving!!!
I start with the phone fully charged and connected to the external battery. You cannot charge whilst it is raining, but by starting with the phone charged and connected to a battery you always have a fully charged phone if and when it starts to rain.
I now have an external battery holder that takes 4x18650 batteries and connectors for USB and magishine type lights. This allows the use of quality batteries (more charge) and means it is relatively easy to carry spare batteries.
I have owned and used a garmin 305, 705 and Bryton 50. I wouldn't go back. I know many Audaxers favour etrex/montana garmins. Personally I can't understand why but I don't decry their use.
More recent Sony Xperia phones have much better battery lives. The author of the ipbike app has reported that he can get 12+ hours using a Sony Xperia Z3 compact with the screen on continuously and will a full ANT+ sensor load.
I plot routes using my laptop usually using mapmyride, but any app that will download a gpx track to your pc is fine. Its then just a matter of copying the gpx to the correct subdirectory on your mobile. Last week , at the start of a 200k permanent, I found that I hadn't copied the gpx to the phone. I just used my mobile to log onto the mapmyride site, downloaded my route to my mobile, moved the gpx to the correct subdirectory and I was good to go.
The two minor downsides I've encountered is that most gloves don't work the screen, but once started you generally don't need to touch the screen anyway except at stops.
In very bright sunshine the screen can be difficult to see, but stopping and shading the screen with your hand is usually sufficient. This is where voice instructions from osmand can be useful.
Ipbike allows you to have the screen on permanently, or to be activated by proximity. It also allows the screen brightness to adjust based on ambient lighting so transitioning from day to night riding is not an issue.
The Xperia Z ultra was expensive when it first came out but can now be had unlocked for around £220. More expensive than a garmin touring, but cheaper than an 810 or 1000. Given the size of the Ultra, I obviously get a lot more use from it than just using it as a bike computer as it can be used very easily as a compact tablet.
I had some issues initially running in airplane mode so now just leave the data connection open. There may be some impact on battery life but I've not noticed it being significant.
I also use the same phone with google maps in the car for 'Take me to x' type navigation. In that scenario the phone battery can last less than two hours due to the data traffic generated by google maps.