I've just started playing Earth 2160, I'm a regular player of RTS games and most don't really live up to what they boast but Earth 2160 does.
I've never played the other Earth games but if they are half as good as this I would recommend them.
Earth 2160 had depth of strategy with simple game play mechanics, the "Easy to play, hard to master" saying is very relevant here. The units are balanced according to factions, graphics are very good for the low requirements. The game-play is fast and stylish and you are reminded of the Sci-Fi element every time to see a battle, it is truly worth buying.