Project Butter uses tripple buffering, vsync and movement prediction, so it seems "smoother" (when in fact the same operations are being done), it also uses more processing for the screen when its needed, so you don't get any lag from transitions and animations.
Along with better RAM use and optimized code (Google always invest some time in it), its a better user experience, but you apps won't load faster, you won't be able to play that 3d game that never could run on your phone because of this.
Most ports don't have full butter effect (specially exynos ones), and RAM optimization of course gives you a boost, but its not a miracle, people saying "gosh, my phone is flying now" are just victims of placebo effect. The screen wont flicker as much, but it will still take the same time to load, and your battery of course will suffer from the speed ups for processing the interface.