I like this puzzle because of the 2 shaped wcurved units. They only move as linked pieces but they appear to seperate with ease but that is not the case.
It is a really nice tactile toy to play with and makes a great worry bead. I managed to reassemble it fairly easily because I had deliberately ensured that I kept one piece orientated the same way all the time.
I have given this to several puzzle friends and colleagues - almost everyone opens it eventually but only the experienced ones are able to put it back together again. Everyone has enjoyed it! In fact, during an operating list, one of my patients (if you are reading this then I hope you are well recovered) was having his procedure awake under spinal anaesthesia and he was quite interested in my little hobby. I gave him this puzzle to play with during his operation. With a little assistance (I think it is harder to do whilst you are lying flat on your back) he solved it and also delighted in the final separation movement.
If you want a nice intermediate difficulty cast metal puzzle then you cannot go wrong with this one.