I've not used one for worms, but I shred paper for rabbit bedding when we have it (eventually it will be composted!). I bought a second hand one cheaply and it is good. Note - get one that has a decent rating - # of sheets they can cope with is of course on a good day and when all of the pages are completely flat - no creases. Each crease is equivalent of another layer.
We had a 3 sheets shredder and it could only really cope with 1 sheet if it wasn't completely flat :(
The one I have now is rated for 18 sheets, so I don't really need to care how flat everything is. It's great for the brown paper that delivery companies use to pack parcels - rip it into 2m sections, fold it 2 or 3 times and then feed it through 😀