I use BlackBerry Messenger as well as Yahoo! Messenger. I could use Google talk as it is loaded on my BB and ready for action. All I need to do is activate it. Right now BlackBerry Messenger and Yahoo! Messenger fit my needs.
I have both but only use Yahoo. I really like the way Google Talk archives your conversations in your gmail account, but I simply don't know enough people who use it. Really, the two programs are so close in quality and function (except for the archiving thing)...