If you're staying for that long I'd say it's deffinitely worth it, check out their site http://www.isic.org/sisp/index.htm
which has in in country/city what discounts you can get. Besides it works in museums and such.
Been to: Argentina, Chile, Syria, Lebanon, Israel, Palestine, Jordan, Egypt. Greece, Turkey, England, France, Italy, Germany, Austria, Belgium, USA, Mexico, Spain, Monaco, Czech Republic, Ireland and Uruguay.
Next trip: Still pondering the possibilities for January 2009... * OH THE MYSTERY!! *
The travel bug is insatiable