Interesting. I hadn't noticed that Star Wars: Empire At War was added to Steam. It includes the main game and the expansion for $19.99.
I never bought the expansion, but the main game was pretty fun. It's made by the former Westwood (C&C) guys and it uses the licence really well.
The space battles are particularly awesome, with swarms of fighters, a few frigates and giant capital ships. And land battles can be fun, running around with Darth Vader slicing up rebels or calling in giant, plodding AT-ATs.
It's a decent strategy game, and it makes great use of its license. No cut scenes or story worth mentioning, though. It shares a lot of similarities with Sins of a Solar Empire.