Finally got mine SP3 after several attempts to switch back to the dark side.
Everyhing positive mentioned about that thing is absolutely correct!
The best thing that has literarily blown me away is SP3's handling of 4K Resolution:
SP3 sits here in its docking station and a 4K Monitor (Dell UP2414Q) is set to extend the Surface's desktop in full resolution and 60Hz AND absolutely no noticeable UI lag whatsoever! Just shocking amazing.
I say that because I came from 15" MBP (End 2013, Nvidia GT750M) and it was impossible to accomplish a similar setup with that machine and scaled resolutions on it and on the monitor, - the UI was way too sluggish on Yosemite. So, the only solution was to use the MBP in the clamshell mode, and working solely on the Monitor (scaled as 2560x1440, which is the second highest setting for that Display).
And now that tiny tablet is actually outperforming that huge machine in everyday tasks by far.
My setup with SP3 is:
SP3: running 2160x1440, 60 Hz
Dell UP2414Q: set to extend the Surface's screen. Running 3840x2160, 60Hz.
UI Lag: not present.
I am officially impressed.