Everything is awesome! Your classic 4x2 lego brick.
To get the nice "click" fit, the stubs on top can be oversized to provide an interference fit with the features on the base. I've included files for interference fits in increments of 0.05mm. 0.1mm seemed to work best on my printer (in ABS M30 on Fortus 360).
Also added an internal 45deg chamfer to the top edges of the inner cavity to allow this model to print cleanly without any support material!
(updated 2/16/17, adding files for 3x2 and 2x2 bricks, also a neat 4x2 brick keychain)
(updated 5/13/17, adding files for 2x10, 2x16 and 1x16 bricks)
I'd suggest starting by printing two of the 0.10mm interference fit models, then move up or down based on how tight or loose the fit is.
The bottom interlocking features are the same in all versions of this model, so you won't need to print two of every type to test the fit.