I like the idea, but they are incredibly expensive for what they are.
I see them as a bridge between rifle and pistol. You already have the pistol, with these kits, you can turn them into a rifle if you need them. The problem is, you can buy a decent rifle for about the same price as one of these conversions. Marlin and Puma lever-guns are all in about this price range, and I would rather have one of those than turn my 1911 into a carbine; you just don't really save anything having the pistol and using these compared to having the pistol and using a dedicated rifle, keeping the pistol for backup.
On the plus side, if you are already set to carry your pistol, these can be a small, lightweight package that doesn't require any extra ammo carry, but I would still argue that I can carry a lever-carbine easier than this contraption. If its just for target shooting, just buy the dedicated rifle.
If they were cheaper, I would consider one.
“There are three reasons to own a gun. To protect yourself and your family, to hunt dangerous and delicious animals, and to keep the King of England out of your face.” - Krusty the Clown