This toy is fantastically designed and engineered. While it's true the assembly is a little tricky to get just right, when done correctly I find that there are no "sagging parts" or problems having non-stop marble runs. As the instructions point out, you really must ensure all the connections are seated properly. This toy worked 100% without a hitch after the first assembly. That being said, I'd say it does take a bit of skill to assemble this correctly. There is no way an 8 year old would be able to do this well without adult help. In our case, I (the dad who likes to build stuff) assembled this myself, which took about 3+ hours.
I must say I am mesmerized by this toy. Watching all the marbles whizzing around the various track layouts is just great. All my 3 kids really do enjoy it; age range 2 - 6. The glow in the dark features are great - marbles, sections of track, and even highlights on the planet cut-outs all glow brightly after a good light-charging. If you happen to have a black light, it's a great effect!
As for un-supervised play, I would not recommend this for anyone under 6 or 7, which is in alignment with the manufacturer's suggested age of 8. Basically if the kid can control the urge to grab and yank at moving parts, then they're fine. I find this toy does require some occasional baby-sitting as parts do tend to fall out of alignment or require "tweaking" over time from getting accidentally bumped or pulled. So it's not exactly a toy you can just "set and forget".
The toy teaches fundamental science and physics principals, design/engineering skills, spacial creativity, and of course some astronomy as all 9 planets and sun are accurately rendered.
This was our first Techno Gears marble set and I already find myself researching other sets to buy and add-on to this one. All for the kids, of course ;)