I was initially concerned about the durability of this product since similar marble runs have been rated as low-quality or wobbly. HOWEVER, my concerns were quickly put to rest when my niece and I began constructing towers on Christmas Eve!
The pieces fit together easily and the bases are fairly sturdy on an even surface (we didn't have much trouble on the somewhat uneven carpet either, however). There are a ton of pieces (moreso than a lot of other marble runs) and there is an interesting variety of spirals, swirls, vortex pieces, spinners and slides.
My niece is 4, and really enjoys putting the marbles in to see where they go (bonus points if they shoot off to nowhere, in her opinion).
This set is designed for older kids if they're going to play on their own. HOWEVER, with adult guidance my 4 year old loves it and does very well with it...and mom and dad approve!
Bonus: This is just as much fun for adults to play with as it is for kids...