I recently reviewed Marbutopia marble run for What2Buy4Kids, we really love it and the boys have had a great deal of fun playing with it. Our full review on their website is here...
Marble Run is a real classic and something that will be played with for years because it grows with your child. It's fun at first to make the simplest constructions and watch your marbles fall to the ground, and then as you get more proficient your designs can become more complicated and intelligent.
It encourages all kinds of development including physical (fine motor skills, hand-eye co-ordination), intellectual (including problem-solving and spatial awareness) and academic (including understanding of speed, friction, gravity).
We give it a 5/5 because it really is great fun as well as being a brilliant tool for learning.