I am not here to courage you to stay away from OZ. I can see people did not give you the specify infomation what t-mobile offer:
Text, Instant, Picture & Video Messaging
Any 400 Messages: $4.99 per month*
Any 1000 Messages: $9.99 per month*
Unlimited Messages: $14.99 per month
NOTE* .10 cents for each text message.
Messaging Overview
OZ Instant Messaging actually uses SMS (text) messages instead of the data - If you select the unlimited messages as its up to you.
Did you know that BlackBerry now supports the following instant messaging platforms: ( go see site
http://www.blackberry.com/YahooDownload/index.do and
http://www.blackberry.com/GoogleTalk/index.do, they using the data instead of TEXT SMS for free!
I believe blackberry will support AIM on BB device in future events soon!
Good luck for the choice you makes..