I have to say this is the best RTS I've ever played.  That's not saying a lot considering I've only played one other game of this genera.  Still I really enjoyed Halo Wars and think it only adds to the Halo story. 

The game has 15 levels and honestly I think the early levels were more difficult than the later missions.  This is probably because I finally figured out how to play the game towards the end of the game.  It's worth waiting and building up your base before you charge into any situation.  Having a large force can allow you to push though most enemies.

I'm going to keep this game a while and play a little online.  Play though the skirmish modes and maybe push the difficulty up in the campaign.  Like Halo, there are several skulls hidden inside each level.  It'll be fun to track those down.