Alright some longer impressions on the puzzles I solved so far:
Marble - level 5/6:
Thats the first one I grabbed after I read a few reviews that mentioned this still being their favourite of the series. Its definitely one of the best looking. Even though the mechanic actually is rather simple, it could take you a bit to figure it out unless you immediately have the right idea. Once taken apart, it turns out reassembling it is actually quite a bit harder, and if you have a friend who wants a good challenge, just show him the final piece and give him the individual parts to assemble them without having deconstructed it. He'll be busy for a while.
I also love seeing it in motion, its a really, really well designed piece.
Cast o Gear - level 3/6:
That one was a lot easier as you only need 2-3 minutes to figure out all the important details and the way to solve it. Then its just a matter of following that solution. Depending on your spatial imagination skills, you'll need more or less time for that. Some people still might take 10min+ for this, even though its not particularly hard. What I really liked was the designer thinking out of the box here, providing a mechanism that still manages to awe when you start fiddling around with it.
Protip: I found this out earlier today when preparing my Cast o Gear for someone else. You can actually increase the difficulty of this puzzle a good amount by giving it a different starting position. The cube has two very obvious dents in it, which are used as a "stand" for the star (not related to the solution). Well, turns out through some positioning you can put the star there on the "wrong side". If you give the puzzle to someone now, they might quickly reach the goal, but on the wrong side. Almost drove a friend of mine nuts today, but if you need another hint how to do this, let me know.
Just for reference, the picture above shows the "standard" positioning. Imagine the star twisted 180 degrees.