Here's Android 5.0 Lollipop running on the Samsung Galaxy S4 - new TouchWiz UI, Material Design in tow
Reportedly, the Galaxy S4 and Lollipop get along surprisingly well - the device seems to be snappier than ever and the folks at SamMobile also report that the memory management has been greatly improved, too. Several UI features from the Galaxy S5 have also made an entree on Samsung's ex-flagship, such as My Magazines and the launcher. Needless to say, all of Samsung's apps have been remade in accordance with the Material Design guidelines.
Unfortunately for the fans of the Galaxy S4, the demonstrated build won't be released to the public as it has quite many an issue on board (Wi-Fi doesn't work, for example) and is “not suitable for a daily driver”, because it's not based on the latest available AOSP build, but on the LRX02E one. As far as release dates are concerned, don't hold your breath just yet - your Galaxy S4 will be officially treated to Lollipop only once all of the newer high-end phones get the newest firmware. Maybe early next year, Samsung?
Done yet? You can also check out our retrospective comparison of all of TouchWiz' versions, as well as our 2014 review of the Samsung Galaxy S4:
Things that are NOT allowed: