My brother-in-law made the switch and he's been an iphoner since the very beginning. He got a Note II though, which is what I have and it's awesome, but I don't see the point of getting it now with the Note III coming out. Impatient ****.
I bought my wife the S4 to keep her away from iphones. She absolutely loves it.
I prefer ES File Explorer as a file manager. Best interface for a file manager I have ever used. Wish they made something similar for Windows, to be honest.
PVStar is pretty good for stream sites (YouTube, DailyMotion, etc). I mostly like it because the stupid ass YouTube app doesn't allow you to listen if you click off the screen. MX Player for videos. Plays every file extension except mkv and it has a tremendous design. I just use the stuff that comes with the phone for music. That and the normal apps like Pandora and what not. For image viewing I use QuickPix, or QuickPic, or something like that. Probably can find something better than it.
The aforementioned Alarm Clock Plus is very good. Use it everyday and it never fails. JuiceDefender to help battery life, Harass Me and Hide Caller ID if you need them. K-9 for E-mail if you regularly use multiple emails like I do. My Backup Pro to backup your shit. Been messing around with Pansi SMS for texting and it's OK. I like the pop up feature when you get a text, but I have some problems with it. ExDialer is good if you want to replace the dialer on your phone. I like it enough that I use it all the time.
But I strongly suggest getting Nova or Apex for your launcher. Combine it with APW, Beautiful Widgets, and Widget Locker to complete revamp the menus and everything.
And of course root your phone before you do anything. All of those apps you can use without root. After you root there's a bunch of better shit.
XDA is probably the best site. Really strong community there.