I use them and love them. The are good for:
-.22's I never use a rod on a .22 rifle any more. Just the bore snake.
-Revolvers--you can clean the bore from the forcing cone end, and clean the chambers easily. Its the only thing I use for .22 revolvers.
-"Quick clean" on any gun. If I have only shot a few rounds, or know I'll be shooting again soon, I just use the bore snake and a quick wipe of the chamber (if its a semi-auto) and call it good.
They're not good for:
-Removing leading or copper fouling. You need a rod, brush, jag & patches for that.
How I use a Bore Snake: I find it helpful to put a bit of CLP or G-96, or other 1-step cleaner lubricant on the head of the 'snake. Then 3 pulls through. I figure that cleans out any residue from the CLP or G-96. Leaves the bore looking nice. But, of course, lead or copper fouling will not be removed. Only powder fouling.