I've been using it on my SGS5 since I got the update in the last days of December.
Pretty nice visual changes overall. It's very fast and fluid but I'd say not as much as 4.4 was in its latest update. I hope the next few updates improve performance even more. Multitasking is slower than before, for example.
Battery life still as great as before and very few other changes. The new mortification system doesn't seem to work perfect always. It's kind of weird. Sometimes I'll get a pop-up window for some apps and for others (like Gmail) the notifications look just like before...