we got this for my dd (now six) for her 4th birthday. It has consistently entertained her! She will forget about it for a few weeks, but usually when she sees her younger dd (almost 3) playing with it, she gets renewed interest in it.
Her favorite cartridge at age four was the disney enchanted princesses one (the pre-K level one). Honestly, she also really likes the drawing game as well as the shape sorter one that just comes standard with the system. She also likes the backyardigans one.
This 'toy' was a great investment. Older dd has loved it for 2 years, and now younger dd is showing interest.