BillCA is right - they will not feed. Even if they did, I don't think the shot would be any bit more effective than birdshot. They are a great aid for training - I started my girlfriend out with them so she wouldn't be immediately turned off by the recoil of a 12 gauge.
Stick with buckshot for home defense - preferably #1.
I've only experienced the minishells a few times and the main thing I love about them is that they are short and you can fit more shells in the mag tube than with regular shells. And no, I don't hunt with them so the 3 shells only law don't apply.
If you have a break open action, and are at constant risk of munchkin or midget attack, I'd keep a hefty stock of minishells arounds. Otherwise, they are just a novelty.