I only have a few GI mags for my 1943 Inland, so I bought a few of the Korean mags a while back for range mags.  Of the 2 I've used so far both failed within 50 rounds.  The locking tabs on the rear of the mags sheared off.  The tabs appear much less sturdy than the ones on my GI mags.  I always thought the Korean mags got decent reviews.  Is this a common problem?  Any other aftermarket mags worth a darn, or should I just pick up some more GI mags?

I've got 5ea 15rd and 2ea 30rd Korean mags I got new from AIM in Feb, 2012, and they run just as good as my original GI mags.  I've tried them in both my '43 NPM and my '12 Auto Ordnance and they work just fine.

I think I got mine from AIM as well, but I can't remember.  I have some of the 30 rounders as well, and the locking tabs on those look great.  But on the 15rd mags the tabs are only attached at the top.  They are clearly split on the sides, and not attached to the mag body.  A little recoil combined with the weight of the loaded mag and the tabs shear right off.  Maybe they just had a bad batch were the tabs weren't stamped properly.  I found a few reports of the same issue via Google, but not very many.  I looked around and I do have 7 15rd USGI mags, so I guess that is good enough.

Someone was taking cheap 15rd mags and packaging them up to pass off as S.Korean surplus. They are apparently junk. I forget what the tip-off was specifically but IIRC it was something to do with the numbering on the package.

mine have been flawless.  I am using them in a newer auto ord carbine though.  I have both 15s and 30s

That might be the answer.

I also read on CMP forum that the latest 'KCI' mags are NOT being made by the same company that was making them for their military. After Korea formally dropped the Carbine the magazine tooling was supposedly sold off, the company making them now is doing so to a lesser quality than original.
Again, supposedly.

Mags for the Carbine have been a sore spot for some time, many have been making cheap knock-offs and even outright forgeries attempting to fool folks. Ive met many people over the years who had troubles with their Carbine and assumed it was the weapon itself, when in reality it was the junk mags. For the life of me I do not understand why someone, especially a quality maker like Mec-Gar, isn't making proper 'mil-spec' 15's and 30's. Surplus mags were cheap and plentiful for decades but supplies have been drying up and people are paying prime prices for reliable mags.

So which aftermarket mags are considered the "best"?  I've seen mags from KCI and PW, as well as Taiwanese mags from Keepshooting.com. Any other options?  I might take a gamble and try a couple more.

I have four or five of these and they function every bit as well as my USGI mags.  One thing I've found out though is that a number of the various aftermarket mags I've purchased have arrived with the spring in backwards.  Because if that, the first thing I do is remove the spring and make sure the higher portion is at the back of the mag.  You can also check them by loading ten rounds and looking closely to see that the rounds are sitting level in the mag.  If they're sitting with the bullet nose higher the spring is reversed.

http://www.keepshooting.com/mil-spec-m1-carbine-15rd-magazine.html

USGI
Yes I know you stated 'aftermarket', but frankly I've yet to find any that were what I would consider reliable. The best I've ever seen were the first batchs of Korean and Taiwanese imports, these were supposedly made on U.S. equipment but then quality started to fall. USGI are still available including NIW, some may look tired and some may even have light rust specs. But cleaned up they function perfect and can be refreshed if needed with new springs.

I've known too many folks that spent enough on trying various after markets and being diassapointed, that they could have just bought USGI and avoided the mess. EB and GB prices are the highest at around $40 but it will save you time/effort tracking them down, otherwise you can tread water and find them as they pop up for $20-25. Your choice.

If 30's are what your after, skip all the aftermarket ones unless you can confirm they are early Korean or Taiwanese from a trusted source. Otherwise you'll have to wade through the mess of fakes, cheap knock-offs, or poorly made examples. Again try to find a trusted seller with good references otherwise just stick with 15's.

If you really REALLY want a 30rd mag, just bite the bullet and pay up for a USGI hardback, but be prepared as one will  likely to set you back at least $100. They were fetching close to $200 but a few small batches were recently put to market and that drove prices down for the time being. They are the only 30's that ive ever used that have been consistently 100% each and every time, even in M2's. Also there are no known fakes due to the specialized (and expensive) way they had to be manufactured (and also why the military switched to split-backs).

My 2 cents at no cost to you.

So far, we're having good luck with USGI mags that I've never done anything too.  I have a few more to test, but the first 3 I've used regularly haven't choked yet.  And some feel more springy than others.  And yeah, none of them are pristine looking.  They all have some kind of finish wear or spots where it rusted.  Since they're all blued and old as the hills.  

I ordered a couple of the Taiwan mags from keepshooting.com to try.  They showed up today and they look pretty nice.  The locking tabs on the back of the mag look much better formed and more substantial than my Korean mags.  They also have a nice blued finish.  I'm looking forward to trying them out.