One of the main reasons that many people decide to flash a rom to their device is the fact that they just can't stand the OEM skin that came with their device. OEM skins can really limit the way you set up your device. There are plenty of ways to resolve this by installing a custom 3rd party launcher like Nova, but if you happen to love the OEM skin this won't be an option for you.
The Touchwiz Os is not that awful especially on the S5 the only real drawback is the lack of space on the homescreen. It comes with a 4x4 grid. With the Touchwiz weather and clock widget and the Google Search bar on the screen there is only enough space for 4 apps. There is a quick fix for this that will allow you to add a few more apps to your homescreen. Brian Tacker has posted a quick mod that is flashable in SafeStrap that gives you a 4x5 grid on your homescreen. This will allow you to use a widget 4x2 widget and 20 apps on the homescreen. The mod also includes a 4x6 grid for the app drawer. You can grab this mod from the link below.
Via XDA