I have used before.
Pros.
If you are a data geek, with IPbike app you can customize and cram as many data on the screen
With ant+ if available, you can pair with power meter etc without the hefty price of, say a wahoo unit.
Cons
Heavier than a bike computer. I have used handphone almost the size a head unit but its still heavier. If you ride fast the road vibration can eventually cause the phone to be dislodged from the mount and risk damaging.
Screen visibility is poor under sunlight.
Battery life can only last 4-5 hours depending on capacity
In the rain the water can mess up the touchscreen, unless you use a screen locking app.
Basically the cons outweight the pros for me. I still use it for navigation alone during bikepacking trip because my computer lacks the function.