I have one for my glock 23 (40-9mm) and it is awesome. Bought it before I bought my G19 but still kept the barrel bc why not? This way can have two G19s at range at once if someone else wants to shoot it. Great for saving some extra cash on ammo if you have a 40sw and want to shoot 9mm.
I even have a factory replacement barrel for my G23 from them that shoots tight. Have i put 1000 rounds through it? No, close to 500 I believe but never a jam. Reason I picked it up was some 3rd gen G23s had barrel bulging issues and got a spare just in case and it's tolerances are tighter then factory.
Because the Frame/slide on a 45 (like a g21) is larger then frame on a 40. going from a G23 to a G19 is easy because frame is the same (therefore can accept g19 mags), conversion barrel overall outside dimensions are the same just has a smaller inside diameter to reduce down to 9mm. The slides are different as in size of hole where barrel goes or else you would be able to just throw a G19 a barrel in but since the openings are different the conversion barrel is needed.