These have also been available on Amazon for a little while - I think there are at least 6 now and they have all been great.

Yes, there are 6 in total now.

What is the magic ending that the Kickstarter refers to - something special when finishing the last piece? They don’t answer, just hint. Is it worthwhile?

Answering it here would take some of the surprise away. But I will say it's totally worthwhile to get one of the puzzles if one enjoys puzzles in general.

The mechanism of the ending seems to be the same in all the 6 puzzles, so you can just get any and see if you enjoy it. Even without it, the puzzle I did (Forest Feast) would have been my favourite out of the 20 or so I did in the last 3 years. The artwork on the puzzle has a ton of small details, which made it very enjoyable. And the unique shape of some puzzle pieces (e.g. a rabbit shaped piece which has a rabbit drawn in the middle of it) added on top of it.

Thankyou! I gather from that that sharing it would be a sort of spoiler, so I appreciate your reply: you answered what I was curious about :)