If you need a kick to boost your UA system processing power, this is the no brainer option. I would guess the other scenario would be to use it standalone to add support for UA plugins to an existing system, particularly useful in a mobile solution.
Unboxed it, plugged it in my Apollo x8p, switched on: it popped up the registration web page. Overall I was done with all of that in about 2 minutes, and up and running. Could not have been simpler than that.
Handling the unit it also feels sturdy, which seems relevant if you use it a lot in mobile scenarios.
The Quad unit is cheaper but I'd recommend the extra investment for the Octo. This is especially true if this is the only UA processing unit you use: 4 DSPs is not a lot, and if you run at higher sample rates you will definitely run out of juice sooner than you like.