Here’s the really short answer to that, that are good and they are better than they were a decade ago.  For those that went through the horror of having to deal with the 1990s and the BS from the assault weapons ban, many of us had to scrounge around looking for pistol magazines that cost 1/4 of what factory gun magazines cost.   I would normally say that you should really stick with the factory magazines that you have for your firearm, but MecGar and Promag do make decent magazines and they are considered in he B+ category.   I would say that the cost of these mags vs factory magazines insn’t significant, but you can get them for a reasonable price.

My Beretta 92FS is probable the most tested out gun I own that has used non-factory magazines.  I even have some no name magazines that hold 18rds each, but I to this date have no idea who made them and they have been extremely reliable.   I had a pistol course a few years ago in New Jersey where I had to leave all of my 15rd + magazines at home and only bring 10rd magazines with me, Promag Magazines ran the course and although I was shooting ball ammo, I had zero malfunctions out of over 300rds fired.   I never got around to testing them out with defensive ammo, but I highly suggest you test magazines with the ammo before you get into your comfort zone.