Still on KitKat (stock rooted) - Benefits to flashing M preview?

Title is pretty much it. I'm still on stock rooted kit kat. I've avoided Lollipop just because at the time it was apparently buggy, and I haven't really paid much attention to roms/android development in a long while. I've been tempted to grab the preview 3 since it came out, but I'm just not sure!

What has improved since KK? Other than having the latest thing, what is good about M?