I just bought a SP4 pen with the nib set and now I am even more impressed. The feel is much closer to that of writing on paper. It is not as slippery; there is more friction which makes for a more natural writing experience.
The pack comes with four extra nibs
B - high friction
HB - medium friction (one installed in the pen, one spare)
H - low friction
2H - very low friction
Microsoft has now made this pen and device SO good. When combined with OneNote, OneDrive, the Edge browser (for annotating web pages), and more, this device changes all of the old rules of computing.