Just bought a 13" rMBP.
Really poor performance all round. I specifically bought this to get more screen real estate.
Unless you have it at exactly 2x scaling, the performance is very poor, especially in Chrome. Safari is a bit better, to be fair, but it's not great.
On a 4K monitor plugged in (w/ the macbook closed so there's only one screen) graphics performance is like a ZX spectrum. Windows don't move smoothly, everything drops down to single figure FPS all the time.
Wouldn't recommend at all. However, I don't want the bulk of a 15" macbook pro. So I don't really know what to do about it.