For those who remember OctroTalk, it goes back to the Windows Mobile days, like the 2008-2009 era. Heck, we worked with the developer on the earliest alpha version.
So it was a pleasant surprise to see them return to form with a Windows Phone edition of the app, supporting Mango no less. The app is v1.0 and does fetch for $4.99, which when compared to the multi-client app IM+, seems a bit high to us.
OctroTalk only supports Google Talk but it is laid out nicely and seem to work a bit better (plus with fast-app resume, handles better than IM+). It supports pre-set statuses, push and toast notifications and even has a flipping Live Tile. The app looks to be a port of the older Windows Mobile version and so isn't completely Metro UI. It also doesn't play too well with those who use GV Max for monitoring their Google Voice information aka GoVoice e.g. our contact list turns into GVMax contact.
All in all, if you need a solid Google Talk app, this is certainly one to consider. But at $4.99, casual users would be best served with GChat or IM+. You can give the app a try here in the Marketplace with a free trial. (Incidentally, we don't know the limitations of the free trial as the app seems to work just fine and in full).