I have just completed a module in commercial law.
Pros:
Less cases to read than other subjects because the focus is on principles and their analysis and criticism
Excellent textbooks in the form of Goode, and Sealey & Hooley. Goode takes you smoothly through the course.
Mostly case law, guided by some statutes
Intellectually very stimulating; something you can really get your teeth into (although this might be a con for some people)
Business orientated; so when I say intellectually stimulating I don't mean philosophising about the meaning of a property right I mean analysing how the principles apply to different facts, and criticising them.
Cons:
Takes time to study each week, requires more attention than most other subjects
Impossible to do when you are tired
Might be perceived by some to be dry (e.g. security) but personally I found it mostly fascinating (except week 1 on the contractual aspects of sale of goods, which was bland).