How can I chat with BlackBerry users?
You can't.  RIM's PIN system and BBM are proprietary.  You can only communicate with a BlackBerry user via PIN messages or BBM if you have a BlackBerry.

Get a new Bold 9900 / 9930, they're great! Full disclosure: I work for RIM. This is not an official statement from RIM either, this is me speaking as a user in my off time at home. I honestly think the new Bold is the best BB yet.