The idea is very good - the tests approach is effective and stimulating; they also provide a range of products to study/experiment with. However, I think that the approach to the learning material is "love it or hate it".

By design, the service doesn't provide any documentation; it provides references to existing technical documentation (of any kind, including blog posts).

Those who expect a focused introduction to each topic will find it very tedious or hard to proceed (for example, the SQLite exercise has important details buried in a very large and confusing webpage), and likely hate it; those who like the challenge of understanding loads of raw documentation will love it.