Earth 2150 is truly a gem for a 3D RTS. It has elements of "epic" stamped over all the campaigns (I had a glimpse of the campaigns from the Prima Hint Book I got at bargain bin prices). There is a lot to do in the game and to think, egads, there's three bloody episodes. And, you not only have to master air, land, and sea forces, but also have to think of "underground confined type" warfare.
The game starts off with beginner (read "tutorial" type) missions and slowly escalates in complexity. There is a bare-bones tutorial lasting 4 missions from the 2nd episode onwards. There are 3 factions in the game with slightly differing tactics to employ ala “Starcraft”. You will have quick access to Construction and Research, as well as a view of the world in 3D ala “X-Com” where you can select your choice of missions. Each mission engages you with detailed and varied mission briefs.
The replayability value of the Earth 2150 trilogy is quite high with some branching missions along the way, and multitudes of maps for skirmish and multiplayer (5 modes of mayhem for up to 15 players in the later episodes). Plus an editor built into each episode of the trilogy to boot, you are set for infinite RTS bliss.
But, it is inevitable that you will notice the downside - there isn't that much variation to the units and structures from each episode, and the landscape colors can be quite drab (well, to be fair, you can't texture the moon with reds, greens, and blues). Nevertheless, these are just minor shortcomings compared to the fun you will have with this trilogy of games.
Thanks to GoG for re-releasing this gem to the masses!