Pro:
1. Build quality doesn't feel cheep at all
2. Screen is so gorgeous. I owned an S3 before &  my S4 with 1080p feel like something from another dimension. Text are crisp, colours pop up from the screen.
3. All action feel really snappy unlike my previous S3 which will stutter from time to time. 2GB of RAM & faster processor really show its power this time.
4. Contact & messaging apps open without delay unlike (again) my late S3
5. I received my S4 with all the latest updates (thanks ayed08). Camera open quickly when i press the icon or from the lockscreen. Image quality is impressive. Low light image is million years better than S3
6. User interface: imho, no smartphone can rival Samsung when it comes to this. Wanna change brightness? U can do it just by pull down the notification shade. On/off toggle button was there since S2 era. Want to go deeper in individual setting? Long press the correspondence button in quick toggle will bring u there. I was getting used to this that i rarely use "menu button>setting" combo anymore. Setting menu is grouped into individual tab which can speed up anything you want to fiddle there rather than conventional scrolling in original jelly bean setting. Swipe left to message,  swipe right to call & direct call are features than i can't live without today. In short i can't imagine using any other phone without any of these features nowadays.
7. Samsung Apps. For me it's one way to get paid apps for free. Yesterday i just download Polaris Office 5 & some paid games (on playstore) for free.
8. You can always turn off other motion & gesture that you don't wanna use. If by any chance you want to show off in front if your friends, you can easily turn it on from quick toggle button (again) in the notification shade.
9. Battery life is superb. Supplied charger also charge my S4 super fast. From 5% to full charge only takes a little more than 1 hour.
10. Gallery loads faster than S3. Ability to drag n drop images & videos to different folder is great.
11. Updates: Samsung is known for its timely & fast firmware update.

Cons:
1. Albeit gorgeous & vivid, the 5" 1080p super amoled screen will suck juice like there's no tomorrow. Prepare your charger or powerbank if you are a hardcore gamers+heavy Internet user like me. Playing Might & magic: Clash of Heroes can bring down the battery from 100% to 15% in less than 2 hours continously.
2. Keyboard. Still no quick comma. Force me to download SwiftKey X quickly after initial phone setup. This is really stupid Samsung.
3. Sadly, tilt to move icon on homescreen is missing. I used this feature many many times in my S2 & S3. I dunno why they remove it. Hope Samsung will realize this & include it in next update.