I think MIG attack jets are my favorite. They come streaking in and look really cool as they loose their payload of rockets at their hapless ground target. Tesla coils are also neat, throwing out blue bolts of electricity at anything that gets too close. The sound effects are terrific. The thunder of cannons and the crackle of machine guns are very realistic.
Music is good as well.
I could listen to it for hours. The manual supplied with the game is excellent. It gives you an overview of every major aspect of the game, including a walk through of the first two Allied missions in the single player campaign. A handy reference card is included for each side, which gives a summary of all units and structures, along with a list of important keyboard commands. The only thing I can think of that is lacking from the manual is a brief discussion of strategies and tactics.