I was going to write a review about the Melbourne Bike Share program, but someone else has already written a great article which already highlights the pros and cons.
Jump over to CycleStyle and have a read.
Granted, the review highlights the need to use a helmet and questions the success of the system. The same issues are being discussed on the Bicycle Victoria forums. Personally I believe that helmets are a must - apart from traffic in the city, there is also the issue of tram tracks. The wheels of almost any bike can get stuck in the tracks accidentally and cause the rider to fall off the bike.
I have seen this happen to fellow riders and almost had it happen to myself.