My son is nearly 3 and has loved this since he was 2, and spends big chunks of time absorbed with it. We have to build the runs for him as the 'run' pieces have a definite entry and exit, and have to be aligned logically with the parts above and below. And he's too young to manage the fitting together. But that's fine. There aren't many 'connector' pieces, so the run tends to end up quite tall and narrow to get lots of marble-running going, (with some of the 'run' pieces being used as connectors instead so they don't do anything). But this keeps it good and compact!
Also I had imagined there might be 'splitter' pieces where the marble might randomly go one way or another, but it's all one way. Only other beef is that it can sometimes fall apart at the seams when my son plonks marbles over-enthusiastically into it, but with older kids this probably wouldn't happen.
Has improved his hand dexterity , and it's also a great game to encourage turn-taking. One of our top toys.