It does more than text messaging. You get little "d" next to the sent message when the message is delivered and when person reads the message "d" changes to "r" to indicate that message was read. I think that is useful especially if you are communicating with busy people so you know they have read it even if they didn't reply.
You can create chats with unique subjects, end chats and start new ones so the history between two people is not just one long chat. You can have many different chats with diff. subjects. You can archive them, email them, look for the by the date etc. Very similar to the way you would use email.
You can also create groups of people for chat.
You can send pictures, voice notes, files, BB maps location, BBM messenger contact or a BB contact to another BB.
It is very reliable and there are no delays with delivering the message. Sometimes it takes 10-15 minutes for my texts to be delivered after I switch my phone on or when I come out of the underground. BBM chat get to me straight away after my BB connected to the BIS server.
Also when the mobile network is congested and you can't send SMS (e.g. NYE) BBM works without problems. During the 9/11 BBs worked (for BBM and emails) as long as you had a data connection. But you couldn't call or text because the network couldn't handle those volumes.
Little piece on security:
http://crackberry.com/pin-pin-messaging-secure