The Rocktopus is essentially an artillery. You move it around like a hovertank and then deploy it in orbit directly over its own tile when you're ready to rock. The upgrade that allows you to de-orbit it at will is very useful, because it's otherwise going to stay in orbit for 10 turns, which can be inconvenient.
It's nice because only other artillery units can attack it, and there's really not much of those, so you can use it to decimate enemy armies with pretty much no return fire from them. Unfortunately, it's useless for sieging cities because Rocket Batteries give cities something like a 4 or 5 tile range for killing orbital satellites, and it's only range 2. If you're okay with saccing the unit or you're sure you're going to take the city, it's fine enough for a quick siege.
Ah - movement 2 means you need magrails or Phasal Transporters or it's going to take forever to get to where the action is.