The Olympus OM-D E-M5 has a decent built-in viewfinder—not great, but perfectly serviceable. Meaning never feeling all that pleasurable to use.
But Olympus has done something nice: the new Olympus VF-4 Electronic Viewfinder can be mounted in the hot shoe of the E-M5, and it works great (be sure to update the E-M5 firmware, it might need the latest version).
I compared the built-in EVF to the VF-4 and it’s no contest: the VF-4 provides a much larger image with far more detail—much, much easier on the eyes.
While it does add a “wart” on top of the camera and does not auto-switch between the LCD as the built-in EVF does, the improvement in image quality is so great that I would never want to shoot the E-M5 again without using the VF-4. It’s unfortunate that it adds a big wart on top (awkward bulge going into my smaller hip pack), but it can be detached and it’s well worth it for shooting.