In all honesty, I had Google Talk already installed on my Samsung Transform and never actually used it. I didn't really see a point when I had text messaging, Twitter, and Facebook to choose from. This has all changed since getting invited to Google+. The mobile app allows you to send messages from your phone, as well as to have other people join you in the current chat for group chat. This is a great way of getting a hold of someone when they can't be at a particular place.
There's also the added benefit of being able to text from either your phone or desktop and then see the corresponding message show up on the same device. For instance, if I start a chat with my friend on my desktop and then need to head out, I can resume and check for any new messages from my phone. Then, when I return home, I can continue the conversation on my desktop again.
Seeing as I've only started using this in conjunction with Google+, I don't know if this was the case beforehand -- either way, it's a very cool feature.
There's also the ability to take a conversation "off the record". This means that the conversation from that point to when it's back on, won't show up in your Gmail chat history. This is only for people with Gmail accounts, but is still a great idea in the case you need to talk about something private without it being found at a later date by someone you didn't want to know about it.