There's a playable demo on Xbox Live, it's free but it's also 1.4GB download so if you have the bandwidth/hard drive space I'd say download it.
I've only been playing Skirmish mode but it is fun, you can either be Covenant of Humans (in the demo only one character per team) and there's only one map in the demo but you pretty much get a base that you have to expand, build vehicles (and yeah, they're the same vehicles from the Halo series), aircraft, and troops. You can build multiple bases if you find an empty site or destroy an enemy base. The controls aren't bad, left stick moves the camera, right stick rotates and zooms. A button selects a single unit, hold A to expand a circle to select multiple units, X puts a waypoint for the units to travel to, LB selects all units. Building is all done through menus, press A on a building site to bring up a menu, move control stick to pick what you want to build. You rely on supplies to build stuff, tech level to build advanced stuff, and you have a limit on how large your army can be.
I like being the Covenant because if you get enough supplies you can build a Scarab tank and those things are pretty much the ultimate unit in the game (at least the demo).