Picked up mine yesterday from O2 on their new "Refresh" contract which is an excellent idea!
I had the Samsung Galaxy Nexus before this and found that even though pure Android was great I liked the overlays of the other manufacturers and I also found that there wasn't any gain by getting a pure Android OS as OK the new updates came first to this phone, generally it was a bit s**t and janky until the .1 update came out which sorted all the niggles and by that point the other manufacturers had updated their overlays to the new version.
Oh and the pure Android messaging app is awful - and I don't want to use stupid GOSMS app or whatever.
You argue that you could use another ROM such as Cyanogenmod or whatever but even 18 months after having the first phone with Android 4 I'm still using "stable" releases of CM10 and not a final release - granted it's better than the pure Android, but even then I could install that ROM on other phones once supported, I don't just need a pure Android OS as a base.
The S4, I think, is brilliant. I'm an Apple fan, I love OSX, iOS etc I just feel that Apple is constantly holding back on putting new technology in their phones I had the 4 and my mate had the GNex and I just found that it was leaps and bounds ahead of my iPhone 4 - at the time there wasn't any 4s available etc.
The S4 is a great step up from the GNex - the bigger screen, the faster cores, the larger amount of RAM, the customisability of the OS is great down to being able to change the fonts in the system which filter through to app (you can change the default font in the system and it also changes it for facebook app for example).
Other bits are just really clever things, the scrolling with the eyes is great, air view (previewing messages and emails without opening them) and the connectivity with other Samsung devices is really good - I can imagine there is some sort of connectivity with Samsung Smart TV's which will be quite cool although I don't have one.
Great phone, admittedly not as well built as an iPhone, but the software is just way ahead of what Apple is offering at the moment - but I'm sure by the time they've brought out the iPhone 6s (yes the 6s because they need to keep up their revenue streams) Apple may have caught up.