BZP (Benzylpiperazine) belongs to a family of drugs known as piperazines which have stimulant and euphoric properties. At doses of between 50mg and 300mg, The effects of BZP are largely similar to those of amphetamines. Based on personal experience, I would list the main effects as: large mood elevation, euphoria, much stronger desire to socialise, more confidence in social situations and a hugely increased enjoyment of music. Basically, if you get some in pure powder form, enjoy it.
After a high which lasts 4-6 hours, the comedown is mild, although some experience slight headaches and nausea. Wikipedia says few people can use BZP daily for any length of time but I would completely contradict that -- just expect to feel bad the day after you stop. Tolerance also increases pretty quickly and it's not abnormal to reach doses of 400mg or more.
Due to its recreational use, its posession and sale is restricted or illegal in several countries. In the UK, it recently became a graded class C drug which is a huge shame because when I tried it in 2008, it was still readily available.
Elliott: Buying 20g of BZP was one of the best decisions we ever made :)