Very easy to use, and very good.

Self powered via a 9v battery, so less cables to mess about with. High quality knock sensor and connectors etc...
I would highly reccomend it. I was almost about to buy the Knockbox, but then this appeared on the scene.

http://www.phormula.co.uk/KnockAnalyserPro.aspx

Ive also used Link's Knockblock, which works well too. Its messier, and more suited to a permanent install. the Link also has a 0-5v output based on knock detect too, which is handy for logging.