I bought 6 KCI mags when they were selling for 5 bucks each. I swapped the springs with cheapest replacements I could find ($4.00 LWD springs). So they're still about half the price of factory mags.
Aside from crappy springs, the only difference that I can see is the KCI mags are slightly shorter than current production G19 mags. When you seat the mag, the floorplate is flush with the grip. Current G19 mags leave a gap. Calipers confirm the difference is over a tenth of an inch. This makes it a lot harder to get the last round in. I have strong thumbs, and I can barely do it. I imagine many people wouldn't be able to do it without the Glock mag loader (and loaders like the Uplula won't fit the last round). And with the last round loaded, the mag is slightly harder to seat in the gun when the slide is closed.
The KCI G17 mags don't have that issue.
If I used my G19 for SD duty, I wouldn't care which mag was in there, Glock or KCI. The only time this gun has ever jammed on me was with the KCI mags with original springs, loaded with Bear laquer-coated ammo. And boy was that a jam-fest. But with the LW springs, they've been 100.00% with any ammo, same as the Glock mags.
I'd buy 'em again in a heartbeat, if I wanted to get 4 or more extra mags. But if you want just 1 or 2 extra, it won't save you much after you add the springs and the double shipping. Maybe you can find a store that sells both KCI mags and quality springs.