I have a ~3 month old 581 series. I bought three ProMag 10 round mags from Midway and have about 600 rounds through them (combined). I'm very happy with them--reasonable price, I'd judge quality to be about 90% of factory, and performance has been 100%. I didn't have to bend the lips or futz with them at all. I would definitely buy again.
FWIW, I had also ordered 2 no-name 10 rd mags from Militaria Press. They were just awful. Total crap. However, Militaria issued me a courteous & prompt refund with no questions asked, so I can't complain about them.
I'd written a reply to this a while back, but must've not posted it before my computer crashed.
Anyway, freesw is absolutely right.
What I'd written is that, in my 580 (non/pre-tapered barrel model), John Masen's have worked flawlessly. But I got those before the factory mags came back on the market.
Triple K's were terrible. They improved considerably with some pliers and a dremel, but they still aren't 100%. The only reason I still bring them to the range is to practice clearing random malfunctions.
I have four Masen 10 round magazines that I bought before the factory 10 rounders were widely available. They have all performed flawlessly for me. I just went out last weekend and used them again in my 581 series and had no problems.
I have 1 factory 5 rounder, 1 john masen 10 rounder, and 1 promag 10 rounder for my 580 series mini. Absolutely 0 malfunctions after 2000+ rounds rotating between the 3 mags. I did notice that the john masen has a polymer coating that is wearing off and I can't figure out how to disassemble the darn thing to clean the interior. The factory mag is probably the highest quality but Promag is not half bad for half the price of a factory mag. Would buy promag again, probably not john masen.