I've gone back and forth on this a few times in the past...
I used to use a smartphone, but then got annoyed at the lack of GPS apps with complex route planning ability (this was a few years back), and the limitations of the waterproof mount options on the market (smartphone overheating is annoying when it's your GPS!).
I then used GPS/sat nav units designed for bikes... first a Garmin, then a TomTom. These are fine, but also hard to plan complex routes unless you use your computer to do so and upload them to the unit. That's fine when you're at home, but if you're on a long trip and want to be able to change up the route as you go, you need to bring a laptop with you on the trip.
So this time around, I've decided that I like the simplicity of using a smartphone. Just bring one device with you on the trip and life is easier. It might not sound like a big deal, but some nights on a trip when getting to a hotel at 11pm after riding 700 miles, the last thing you want to do is pull out your laptop, laptop charging cable, connect to wi-fi/ethernet, plan the next day's route, upload that to the GPS unit, unplug everything, put the laptop away. A 5 minute job can seem like a herculean task after an extreme high mileage riding day.
Smartphone GPS software has come on a bit in the last few years too, so plotting complex routes on a smartphone is achievable these days I believe. As have waterproof cases and mounting systems.