I haven't tried any of their slugs but I know they make them because I seen them at the local store where I picked up a box of the buckshot ones to give a try.
My results were like many other people in my Mossberg pump, they are short enough that they can turn up end wise or even completely around the wrong way and don't chamber. I did shoot a couple by dropping them straight into the chamber and the recoil was quiet low. I kept the rest of them and maybe one day i'll shoot them in the Winchester 101 over and under.
As far as the price of them vs load your own, I load my own standard buckshot loads but I use a standard mec 600jr and it won't come down far enough to crimp that short of a shell. You probably could load them cheaper if you desired to shoot many of them. Then again reloading doesn't save you money, it just lets you shoot more with the same amount of money you would have spent anyway. With a shell trimmer and a roll crimper tool you could shorten up your own shells. Good luck finding any load data for a mini shell tho maybe its out there, I never bothered to look.
I think I give right around $12 for a box of 20 of the mini shells tho, I have wasted way more money on way less useful stuff before.