When it comes to custom UI skins that are designed and developed by smartphone manufacturers, they tend to get mixed reviews from owners of those devices. Samsung's TouchWiz UI is perhaps the most infamous, getting reactions that tend to sway in both directions with people loving the setup or hating near everything about it. Despite your own perceptions of how the software performs, looks, and functions, it is most commonly associated with lag issues due to the immense amount of resources it draws from the Samsung handsets, even high-end ones. I personally don't care for TouchWiz and never have, but even someone who isn't a fan of it can admit that it has come quite a long way from a year or two ago, and with Android 5.0 Lollipop it actually looks fairly clean and something I could work with.
As Samsung has yet to start pushing Lollipop to its devices, we can't really attest to how well it performs even against the version of TouchWiz that is currently sitting on phones like the Galaxy S5 and the Galaxy note 4, but when you compare the UI design and style, it's honestly leagues ahead of older versions. Things will appear to be a little flatter in TouchWiz with Android Lollipop, and there will of course, be quite a bit more color with deeper header bars and such. On the whole from what we can see in these images, TouchWiz finally looks appealing. Much of this should really be attributed to Material Design though. With Material Design, Samsung is now able to pull off a pretty good looking UI that looks somewhat like stock Android without giving up their own style and flare.
TouchWiz with Lollipop is still TouchWiz, and you'll notice that if you're the owner of a Samsung device once you get the update to Lollipop. You may even end up not caring for the design changes, but as a majority, we think people will enjoy the way things have changed. Samsung is still going to be using many of the same icons and things from TouchWiz so if you are fond of the way their icons and menus look, that isn't completely going away, they're merely being uplifted, or elevated to a level that only Lollipop and Material Design could really help TouchWiz accomplish. Perhaps now going forward with Samsung reaching a point where people hopefully won't hate the UI, they can spend a little more time focusing on optimization of the way the UI performs, and finally get rid of the complaints about lag.