I really don't think vanilla android is any smoother than this phone. This phone as is, is as smooth as the galaxy nexus. If not smoother. As long as the hardware is this good and is up to the task, it will be smooth.
IMO... I do not think vanilla android is all it's cracked up to be. It's plain and while this will probably get me slammed, ugly. The launcher is about worthless as far as customizing. As for touchwiz, while not my favorite launcher, this new version is far better than Google's stock android launcher.
I absolutely hate what Google has been trying to do with on screen buttons. Absolutely hate them and am happy Samsung took the stand and didn't put them on this phone. If they had, I wouldn't have bought it.
Yes this will most likely cause updates to roll out slower. But I like many of the bells and whistles this phone has. While many of them I can do without, there are some that I use almost daily. All of them would be gone if it was stock android. So from a functionality standpoint, I don't really mind that we will likely have to wait longer for the updates.
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Just Flash It !!!