One thing Ive found with conversion barrels, and just about anything that "converts" in general is, you usually settle on a combo you like, and everything else just sits in a box or on the shelf doing nothing. In some cases, the price of the combined conversion parts could be another "actual" gun that might actually get used.
My experience with the spare barrels has been, they all dropped in and worked fine, and I never had any function issues with them installed, but only one set up really ever got used once I found what I liked.
I had extra .40 barrels for my SIGs in 357SIG, they worked fine, and they basically only got shot once to prove it, and that was about it. On the plus side, I about doubled my money on the extra cases of .40 ammo I bought to go with the barrels when I sold them all. Did even better on the 357SIG.
I had a Lone Wolf 9mm barrel (40-9) for my Glock 31, it worked fine. I ended up using it more than the factory barrel, and there sat the factory barrel.
I also have a Lone Wolf threaded 9mm barrel in one of my 17's that has a suppressor on it. It too works great. The factory barrel sits in the safe.
If you have a purpose and they fill it, then they are probably worth the money. For me, the 9mm barrel in the 31 was a good thing since the 31 was being beaten to death by the 357SIG. It also convinced me that I was just wasting money on 357SIG, as +P+ 9mm basically duplicates it, and doesnt beat the gun up like the 357SIG. Its also a lot cheaper to shot and reload.
The threaded barrel in the 17 is more of a specific need thing. At the time I got it, I didnt realize Glock made factory threaded barrels (not that Ive seen Glocks for sale with them), and in the long run, it turned out the better choice, as the Glocks thread is metric, and not as common as the LW's 1/2x28, which will go on more guns without extra adapters.
As far as resale, the extra barrel really didnt bring any extra when I sold the guns that had them off. They sold pretty quick, but they also only sold for a few bucks more than guns that didnt have the spare barrel.
Personally, except in the case of the threaded barrels, which address a more specific need, if you want a gun in a different caliber, youre probably better off just getting another gun, and having two that you know will work.