http://www.skepticnorth.com/2010/08/...ype-or-hope-2/
This gives a good overview if you really want to get into the science. The main points were covered pretty well before.
It is likely better to just donate it to a bank. The chance of actually having to use your own babies cord blood is extremely small and the $3000 (total cost over 18 years) is likely not worth it.
Sure it could come in handy one day, but the current evidence points to it not being that beneficial.