Personally, I'm not fond of Halo Wars for a number of reasons. One, for being the first truly mediocre Halo title, and two, for resulting in the loss of one of the finest RTS developers, Ensemble Studios. However, if you're less irked by all this, then why not pick up Ensemble's last effort from LoveFilm, available for just £6.93.
Averaging an 82 on MetaCritic, Halo Wars impressed critics with its streamlined RTS controls and accessibility. Personally, I was severely underwhelmed by the game. It's supposed to be a Real Time Strategy game, emphasis on strategy. And yet, for some reason, Ensemble decided to funnel entire armies through gauntlets of Covenant-overrun environments. You can't mine resources, or plan well-coordinated assaults. It is, essentially, a Halo game from an RTS perspective, instead of an RTS from a Halo perspective. And yes, I'm aware how backwards that sounds!
Set in the first years of the human-Covenant war, Halo Wars tells the story of one ship, the Spirit of Fire, and its crew of pilots, soldiers, scientists and, yes, Spartans. Following a Covenant attack on the planet Harvest, the Spirit of Fire and a Covenant fleet stumble on a Forerunner Shield-World, where the elusive architects stored an army of Dreadnoughts, ships with nigh-unstoppable power. The Spirit of Fire crew realise the Covenant seek to claim the Dreadnought fleet for their own, and know if they don't stop them, the Covenant will win the war.
It's an interesting story, although yet another reminder of how branching out series often results in a dilution of the original canon. Finding the Halo ring in the first game was supposed to be a seminal moment in Halo history, and yet now we find out the UNSC and the Covenant have already set foot on Forerunner land.