I rode about 18,000 miles around the country this summer and carried a Ipad3, a Samsung GS3 (for part of the trip after killing an Evo) and eventually a Garmin Montana.
Streaming music to a bluetooth helmet is great, but most don't have the controls to click forward or backwards between songs. Just manage your playlists well.
What I discovered using all of the devices is that Google, Apple and Garmin ALL have map problems. MANY times all 3 would disagree about routes, roads and even the existence of roads. Garmin is actually the worst!
The big downfall of all of the phones/tablets is A) sunlight readability B) true weather and shock protections C) Usability with gloves on! The maps and search functions were better than the Garmin though.
The Garmin is weather and shock proof, easy to see in the daylight, works awesome with gloves on and has lots of great trip data options.It also locks to the bike and is powered by it. It's way over priced and has some serious routing flaws, be careful
Damage a phone and you are without a phone and emergency communications. I learned this when I got the Evo wet in the rain forest.
9 days of riding with no phone