It's a really good way to get a good overview of the field, and what some of the problems have been already solved in bioinformatics. It's definitely best to combine it with a more traditional textbook such as "Introduction to Computational Biology" (Haubold and Wiehe) or "Biological Sequence Analysis" (Mitchison, Krogh, Durbin, and Eddy).
Hope that helps!
Rosalind is a great way of improving your knowledge of biology and CS. It is quite cool to see computer science and mathematics problems and techniques applied to bioinformatics.
I think I have a new hobby!