I had an older KMKII-512 and sold it so I could buy a new KMKIII-512. The finish, weight, and balance are identical. The features I wanted were the new style magazine release button that works like a 1911 and the scope mount. I really enjoy these features and for me, it was well worth trading because I detested the magazine butt release on the MK I and MK II.
I think many people are overly sensitive to changes ... especially if they are safety related. I really didn't have an issue with having a magazine disconnect because this gun will not be used for self defense. However, I did have an issue with the "Band-Aid" engineering Ruger did to modify the MK II design. The magazine disconnect lever drags on the magazine so when you press the release button, the empty magazine does not fall free like a 1911. I removed the magazine disconnect, spring, hammer, and hammer bushing and installed a MK II hammer and bushing. It now works the way it should when you press the mag release button. BTW, this modification makes it easier to disassemble because you don't need to insert a magazine to dry fire.
My next problem came when I tried to field strip the MK III. I don't have an issue with having the lock because it is out of sight and I would not use it anyway. The phony lock is just a screw that interrupts the mainspring strut and again is a classic case of Band-Aid engineering. Rather than redesign the gun for a lock, Ruger modified the existing mainspring housing and Safety Switch. With my MK III, I could not remove the mainspring housing without farting around to center the lock screw. Additionally, the safety switch was flaky and did not hold the sear tight like in the MK III. Another major issue with the lock .... before you can lock the gun, the hammer must be cocked and the Safety switch must be in the SAFE position. This doesn't make good sense to me. It should lock with the hammer down. Fortunately, it only takes a few minutes to remove the lock. I also installed a MK II Safety Switch that holds the sear tight, like it should.
At first, I did not like the loaded chamber indicator (LCI) but after shooting my MK III quite a bit, I got used to it. I like the fact that you can "feel" the LCI to detect a loaded chamber. My only complaint about the LCI is ... it's totally fugly. FYI, the early production MK IIIs were recalled for the LCI. Seems the solid metal ones on the first couple thousand guns would cause the gun to fire if the LCI was struck or the gun was dropped and hit the LCI. Again .... more Band-Aid engineering. The current spring loaded LCIs work fine but I think Ruger engineers could have come up with a more cosmetic solution.
So now I call my MK III a MK 2.5 because it has the best features of both models.
Just a few minor corrections to the above posts. During the last few years of MK II production, all adjustable sight models WERE drilled and tapped for a scope and included a Weaver style base.The Slabside MK II models had the Ruger steel base that works with conventional Ruger steel or stainless steel rings. Not all MK IIIs are drilled and tapped for a scope .... only the models equipped with adjustable sights.
Here's my MK 2.5 with the Ruger steel base and rings. BTW, this is one shooting machine. The only gun I've found that is more accurate is my $1200 S&W Mod 41.