I use phone GPS apps in areas without phone service. When it comes to dead zones, no more problem for phones than dedicated GPS devices.

My Samsung Galaxy S5, if the screen is on, GPS active, navigating to a destination, using the cell network, and charging with a 2a car charger, my phone will not keep up with the power draw. It isn't a fast discharge, but over a 4 hour period it will drop 10% while hooked up to a charger.

A lot of the draw is the screen brightness so if it is a bright day outside the screen automatically adjusts to a brighter setting, thus using more power.

Yup, this is not uncommon. I got this message with my old phone when using GPS, also, that the phone was discharging faster than it was charging. I haven't actually charged my GS6 while using GPS, since it's generally been shorter drives, but wouldn't be surprised to encounter that situation with the slow discharge.

Google Maps and Here Maps, for me. I've not needed to try it, but Here Maps allows download, so will work outside of coverage (I *think* Google Maps will, also, thanks to recent enhancements).

I find it amusing that some people will drive off a cliff, if that's what Tom Tom tells them to do. I've seen some terrible driving due (presumably) people being a little too trusting with their tech....

"What do I do now, Tom Tom?"
"At the next set of lights, check make-up in rear view mirror. Then turn left and drive off edge of cliff."

The same people that would drive off a cliff if Tom Tom told them to would also do it if Waze or Google Maps told them to.

Literal people in general, in other words.

I think which device is best really kinda depends on exactly what the use for it is going to be. If your use is mainly driving on maintained roads and walking around cities, a phone might be perfectly reasonable for what you're doing (and I'll admit, I use that a lot more these days than I used to). But if I'm going off and doing any off-roading, especially in teh mountains or something, or if I'm doing any real hiking, I'd rather have a dedicated gps unit, preferably one that runs on regular types of batteries instead of having a built-in battery like a lot of the auto ones do. Battery life is a hell of a lot better than a cell phone that's using gps, and it's easy to have a few spare AA's to throw in if I need to. Personally I still use an old Garmin 60csx for a lot of things (although in general not as much for navigating for a car anymore because the maps are getting older, I may have to replace the unit eventually). I find it's also pretty useful for just keeping a track log for things, and using that to sync photos to gps tracks for my slr.

I do also kinda wonder about the long term health of using the phone for a gps is going to be (for the phone). It tends to really make the thing run awfully warm, which in general isn't great for electronics. In the summer it's not a huge deal because I can just use the vent mount and run the a/c, in the winter that doesn't work quite as well :). The garmin unit I have is also a lot more on the durable side than the average phone is. Rain, heat, cold, nothing's really phazed it. (Well, swimming in the ocean with it really isn't a great idea. It's waterproof, but not *that* waterproof. I had a 60cs before the 60csx.)

Personally I also find that phones tend to have far more trouble acquiring and keeping the gps signal than a dedicated gps unit tends to. For me at least, I've found that sometimes the phone gps can get a little quirky on finding its location properly.

But anyways, like I said, what's best does depend somewhat on what you're planning on using it for.

Google Maps has been pushing me to download offline maps for my home and work areas, though you can download for any area

Google Maps has full offline support on Android.

Pocket Earth Pro on an IPhone does not require wireless access and can operate off of the GPS on your phone or can operate even more efficiently using an external GPS like Bad Elf. You can download maps at no cost from nearly anywhere in the world and if you have access to the web it can search for locations etc. Can be used at 30,000 feet if you have a sensitive GPS like Bad Elf which communicates by USB or Bluetooth to an IPhone.

Waze. There is no substitute.

I presume that the behavior is the same if you're on a network that doesn't support simultaneous voice and data and you get a phone call, right? Luckily I haven't had to find out thus far.

not in all countries

Yes, I'd assume in that case that it would continue on the existing route, but might or might not offer any re-routing. I'm a bit surprised that canadiancow said it has full offline support; as far as I knew there was limited offline re-routing support and simple cases but couldn't calculate complex routes even on cached maps.

Perhaps it's gotten better. Which networks still don't support simultaneous voice and data?