Currently have an iPad 3 and the wife has an iPad mini. No immediate plans to upgrade either one as they're both meeting the need. However, the iPad mini retina is starting to pique my interest for future travel. So who knows, things may change!?!?!
I use it all the time, Starbucks, United and the movie theater. It's easy to have it on the lock screen so you don't have to unlock, go to the app, open it then use it.
Since I'm on TMo I'm jumping to the 5 (even though I currently have a 4S) since it allows me access to Tmo's HSPA+ network as well any of Tmo's refarmed 3G networks.
You can add it later. There's no requirement to buy a service when you purchase it. However you do need to ensure that you purchase a model that is compatible with the service you will be using.
Re: *Will the iPhone 5S make you switch to the Samsung Galaxy S4?
Definitely not. Being a former Android "Fanboy" I was initially thrilled with the prospects of being able to customize my phone into oblivion. This thrill soon gave way to the fact that I was spending more time tweaking my phone...