Absolutely fantastic subject, the brilliant continuation of
COMP20004 Discrete Structures. Topics covered (undecidability, decidability, complexity theory, time complexity, space complexity) are very very very very very (5) interesting, and Harald is a great lecturer. This subject provided me with a much appreciated change in style (proofs are usually intuitive, not a lot of bashing), which is appreciated as a math student.
The tutorials are done in a group-ish setting, so a lot of fun was had discussing the various problems provided to us. Also the assignment style is fun - Harald gives super hard assignments (and a lot of time to do them), and its a satisfying experience to finally realise the correct way to do something after 3 weeks of thinking about a problem. The class is super small (<15 people) and it has a nice atmosphere. If you can take this subject, do so.