I have a single Mec Gar mag for a Ruger Mk II. The button to depress the follower spring is slightly smaller then the factory Mk II mags. That makes it just a little bit harder to load the mags. Not much, but enough to notice to where I can tell the difference between the Ruger mag and the Mec Gar mag as soon as I start to push the button down. Other than that, it's worked fine.
I snagged this mag when it was deeply discounted at a local store. It was either mis-priced, or on clearance. It was substanially cheaper then a real Ruger mag. If I got another great deal like that, I'd buy another one, but for what they charge for full retail I'd rather spend a little more and get a Ruger factory Mk II mag.