Not all aftermarket "conversion" barrels are "drop-in" - some require hand fitting by a gunsmith. Lone Wolf is a true "drop-in" conversion barrel.
Most often, with other models of guns with conversion barrels, people have problem with extractor not grabbing enough of the 9mm case lip to extract reliably. I run Lone Wolf 40-9 conversion barrels in my G22/G27. 40 Glock extractors won't fully grab, but will grab enough of the 9mm case lip to extract the case. I have shot tens of thousands of rounds through the LW conversion barrels and they have worked very well.
As to magazine, newer 40 caliber Glock magazines mostly work with the conversion barrels, but the older magazines have some feeding problem (maybe due to the magazine follower design/tension of the spring). If you run into feeding problem, you can try changing the follower/spring or just use 9mm magazines. I run my G22/G27 with new factory magazines with no feeding problems.
The Lone Wolf barrels are match grade and have conventional land/groove rifling instead of Glock's hexagonal rifling. The chamber is just about the tightest I know compared to other factory barrels and the case support is very much improved over Glock's loose chamber/deeply cut in ramp area. For reloaders, this stresses the brass less and eliminates the dreaded bulged "Glocked" cases some people have hard time resizing. The better supported chamber and the conventional land/groove rifling is better if you shoot lead bullets.
I use the LW 40-9 conversion barrels to transition shooters from 9mm to 40S&W and the conversion allows for much cheaper practice option, especially if you reload with lead bullets. You do not need to replace the recoil spring as they are the same for 9mm/40S&W. I load my 9mm fairly hot using Winchester jacketed bullets to mimic mild 40S&W loads. For plinking/range practice, I use 125gr lead bullets with moderate recoil.
So, is there any down side to this $99 (Cheaper Than Dirt) conversion that gives you match grade stainless barrel? The only thing I can think of is that depending on the powder charge, the 40 Glock extractor will leave a small dimple or indentation on the bottom of the case. So far, I have not had any issues with this. Kinda like having a built in reload stamp into your 9mm cases - count the dimples to see how many times it's been reloaded.