I decided to experiment with Android and the S4 after having iPhones since the day they were released. It's only been a couple of days, but my quick initial impressions:
PROS:
[1] LOVE the bigger screen. No problem using it onehanded or putting it in my pocket. Apple really needs to revisit their screen size
[2] LOVE that I can choose my preferred apps for browsing, email, SMS, etc. It's ridiculous that on my Mac and can do this, but on the iPhone I cant.
[3] Customization possibilities are amazing. (See cons).
[4] Keyboard and text input is VASTLY superior to the iPhone. Love that you can't choose from a number of different possibilities.
CONS:
[1] Don't like how notifications are displayed. (I know that Android did this before Apple, but Apple definitely improved on the notification panel, making it visually appealing and organized).
[2] Miss app badges; the gazillion of teeny, tiny white (and frequently obscure) status bar icons are a mess, all crammed together up there.
[3] I find almost all the motions and gestures that Samsung hails as making the S4 unique are inconsistently deployed, largely gimmicky, and frequently don't work.
[4] Too many customization options and inconsistencies. Okay, I LIKE being able to customize things, but I do miss the across app/platform consistency that Apple is known for. Definitely makes for a more predictable experience that doesn't change radically depending on what app you happen to be using.
Anyway, just my 2-day old experiences. I'm sure these will change as I get more experience with Android and the S4.