I'm loving Sins of a Solar Empire at the moment. I had a 3 player coop game last night with some friends. The computer were set on difficult. The AI is very difficult to fight, especially when you set them to aggressive. Its a very rewarding game, the visuals and effects are excellent. If you like space battles then I suggest you check it out.
From a modding point of view the game is quite configurable and based around several sets of text files. I'm looking forward to seeing the upcoming Rebellion expansion.