All I run are OS mills (with the exception of the tmaxx, but that truck is getting replaced soon). I've had a CV .12, CVR.12, RG.21 (that one blew up due to age)... Those engines were more than rock solid. The CV.12 hung with CVR.12's on a straight run. The RG was hanging with engines that were a step up and above it.
I now have a VZR-Turbo II and a TZ .12 (which I haven't finished breaking in yet.. darned rain!!!), and the little that I have gotten to run them, I love 'em. I can't wait to see what the new breed have in store for me, once these little monsters are fully broken in.
All the OS engines I've had have held a tune better than Enrico Pavarotti can ever hope to, been more powerful then Rosanne Barr at an all-you-can-eat buffett, and been more reliable than heartburn after a Taco Bell midnite run.
I've had Ofna/Piccos, Megatechs, and SH (GS) engines, and ya just can't go wrong with OS. Reliability and power, all in one package. The one time I had a problem with an OS Engine (aside from the RG going BOOM) was a cracked piston in a VZR-II that I ordered. OS took it back and shipped me a new one, all on their dime. Can't beat that customer service at all.