Gotta be honest, I've never handled any Nikkor lenses, but I'm sure they're also excellent. Nikon and Canon didn't come to dominate the market by building crap gear. It just so happens I was born into a Pentax family and never had any reason to drop the coin on a new body and new lenses at the same time.
That said, I almost bought an FM3a after college just to support the complete lunacy of building them new. At that point, I didn't really have enough Pentax lenses to be wedded to the system. I still kind of wish I had.
You got me thinking though: since you can adapt basically anything to E-mount, the cost of trying out another line of lenses is now down to <$100 US (cost of adapter), at least for manual focus and aperture lenses. A well-designed mirrorless mount and adapter basically renders the concept of a photographic system obsolete.
For those unfamiliar, the key dimensions on a mount are the flange distance (distance from the plane of the mount to the plane of the film), and diameter. There are two things to know about flange distance:
1) It used to be "large" (45mm give or take a few) to allow for room for the mirror in a DSLR to swing upwards.
2) You can't adapt a lens with a smaller flange distance on a body that has a larger flange distance. Well, you can, but you use infinity focus.
Because mirrorless cameras don't have a mirror (go figure), they're freed from constraint 1, and can make the flange distance as short as they care to, basically letting them eliminate the issue of 2.
Adapters used to be awkward, at best, (nonexistent at worst) because they would have to adapt two mounts of fundamentally similar diameters without having a lot (any?) room to move the lens along its axis.
Adapters for a mirrorless body can offset the lens tens of millimeters along its axis from the body flange, leaving plenty of room to resolve any mechanical incompatibilities in the mounts.
