I have a Lone Wolf 9mm conversion barrels for both my Glock 22 and Glock 35. (and a Storm Lake conversion barrel for my Smith & Wesson M&P in .40 cal, and a Bar-Sto conversion barrel for my Sig 226-DAK in .40)
I did it because 9mm ammo is cheaper than .40 cal ammo. I have no plans to use the 9mm barrel for defensive use.
I did read that Lone Wolf had a bad batch of barrels that were slightly out of spec, but I haven't had any problems with mine. I haven't shot them a great deal, either. But so far I'm quite impressed with the accuracy.
I use 9mm magazines with the 9mm barrel.
I did not have to change the ejector or extractor.
BarSto and Storm Lake and KKM precision also make conversion barrels.
I think a 9mm conversion barrel is well worth considering if you have a Glock in .40 cal. It would allow for cheaper practice, an another option should ammunition availability become an issue.
(I also have an Advantage Arms .22 conversion unit for the Glocks and a Sig .22 conversion unit for the Sig)