This one comes up pretty often.
Garmin has a very strong following and has evolved is to fine solution.
I’ve been using my smartphone for many years which stared out as a pain but now it’s a great option. I think it’s very telling that the ‘new’ features on Garmins are now playing catch-up to smartphones.
One of the great things about smartphones is you can run many apps and use different one’s depending on what you're trying to do. Most are free or cost very little so you can experiment.
I have an iPhone and this is what I use:-
Strava :- I use this to track my rides and display cycle computer type information (speed, distance etc). I use the app as I use Strava, it decent but not ground breaking but makes uploading to Strava a one button process. I have also bought the Wahoo Fitness RFLKT (Bluetooth second screen), I love this and it works great with the Strava app. It also allows me to free up my Phone screen for pure navigation.
MotionX :- I’ve been using this for years now. It work very well and I use it to navigate. I like to plan my routes in advance (using the website gpsies). I navigate by following my route (line on the map) as a scrolling map. I prefer this approach.
TomTom :- I also have TomTom (my phone is my car satnav). I very rarely use turn by turn navigation on the bike but when I have the cycle setting works a treat.
Bluetooth 4 (low power) will replace ANT+ over time. It’s technically better but the fact it’s an industry standards it’s a given. If you have an iPhone 4S or later it will work with Bluetooth 4 devices. Both Topeak and Wahoo do a range of heart rate monitors, cadence sensors etc.
The 2 issues with using a smartphone are:-
1) How do you mount it – There still haven’t made the perfect phone mount. I use the Topeak Ride CaseII for iPhone 5, it’s about 90% there as a solution for me.
2) Battery life, if you’re riding all day – This is pretty easy to solve by plugging the phone into an external battery. My old solution was £20 battery bought from amazon left in a frame bag, worked a treat. I have recently bought the Topeak Battery Pack. This velcros to the stem. I’m very pleased with the Topeak, it looks a lot neater but it’s a bit pricey at £50