The Earth series are part of the classic RTS genre, meaning you send a few units against a few enemies and they shoot at one another slowly, slowly chipping away one another's health bar and having none of the scale or realism of a real war offensive. On top of that, the acting is terrible and this is a fairly story-driven game with characters so you hear a lot of it. The campaign feels unbalanced because at times the missions are very easy and at other times the missions are nearly impossible, though the latter has been my experience with many other RTS games, so perhaps it's just me. The AI is very average. Also, a weird design choice done here, that has also been done in a number of other RTS games, is to force the player to play through the campaigns of all the factions in order to complete the story.
Having said all that, there is something interesting here. The factions look and function quite differently from one another. Also, instead of building different buildings for different purposes, your main base expands to the sides. You can also conduct research and upgrade your vehicles into different types.
The graphics are pretty good, in fact, probably this game's strongest suit. The stark shading combines well with shiny metal. The faces of the characters are distinguishable. There are pretty lava flows and colorful explosions and lasers. There is a day-night cycle. The game even features a sky fully equipped with stars and moving clouds.The game acknowledges its visuals by letting you zoom in all the way to a first person perspective. A real disappointment is then that you are not given control of your weapon in that view. This could have totally been a RTS/FPS morph if the developers tried. Character animations are not great though.
So, with its good variety and good graphics, this strategy game rises just above average for its time.