The MK512 was my first .22 pistol and I've gotten a lot of good use out of it. Especially in the past year. I got the KMK678GC in August of this year and put a Leupold M8 2X scope on it. The 512 still has just it's iron sights and that's what I use mostly for 25 yard shooting. The scoped 678GC is what I use for 50-100+ yard shooting (not that I'm that good. I just DO IT!). Even with the cheap Federal 550 round value pack ammo both are very accurate!
I love both of them and wouldn't want to be without either.
Jeff