A bit challenging, but very rewarding. Some of the pieces are very easy to break, so be very careful. Have at hand some superglue (Loctite or similar) especially for the metallic paper adhesives (which are, surprisingly, easier to mount than I expected). For the gears some kind of grease is advisable, also (I used lithium grease, but any other thick lubricant will do). Definitely not for beginners, and that's the only reason I give 4 stars.