The Norinco mags are actually VERY VERY good. As good as USGI, and I don't say that lightly. I will add a caveat though, I'm talking about the Factory 356 mags in light grey parkerizing with a light grey parkerized follower.
There is another batch of Norinco/Polytech mags being sold these days with a rivet in the spine and a blued follower. These work, more or less, but are nowhere near as good as the parked-follower mags. The bodies are a little too wide, so they go into receivers hard, even Norinco receivers, and the followers have improperly formed BHO's, so they tend to not lock back the bolt after the last round.
The Checkmate mags are OK. If you can find the ones in 1990-1993 USGI packaging (or earlier), they are really good. The more recent productions mags from Checkmate are a little hit/miss. Many were made for the commercial market and don't live up to government contract standards.
These days the only CMI mags I see around are the 5 round shorties, I have a few and they work well, but are limited to 5 rds, not 5.5. You can only insert them on an open bolt.
Also worth mentioning, for the first 10 years or so, Lever and Marstar were the only importers of the M305 and all the mags were follower limited and could be disassembled for cleaning. Most mags in Canada are of this variety, but they are awesome mags and nobody ever wants to sell any.
Later, around 2009, Bell Lifestyle, North Sylva, and others started bringing in M305's and mags. At this point mags became hit/miss. Some started to be riveted, others have un-necessarily welded floorplates and limited followers. Of those currently for sale, the welded floorplate models with parkerized followers are the best mags at retail.
Recently I buy the welded mags, cut off the welds and re-park the mag bodies and baseplates in Manganese phosphate. Then they are still limited to 5 rounds (limited follower), and can be disassembled for cleaning. They also look identical to 1960's USGI mags with the darker parkerizing.