The DS really isn't the appropriate venue for 3D action games. We've seen plenty of admirable tries (Splinter Cell, for one), but few hold a candle to their console versions. MechAssault: Phantom War looks like it'll fare a little better, though, mixing mechs, tanks and hacking skills into a game that actually has not-trash 3D graphics.
You'll be hoofing it through various missions, most of which involve blowing stuff up as a towering, walking tank. Our first task was to out-shoot two incoming enemies - a pretty easy task, as the targeting reticle was very forgiving and gave great cues to start pumping missiles out. Once they were gone, we hopped out of the current mech and commandeered another one, prepped and ready nearby.
Once in the other mech, we noticed each one came equipped with its own weapons and attributes - pretty standard, but without some differentiation, this would get old very quickly. Combat with other mechs was a little more interesting, as you can all fly, hover or chug along the ground, blasting artillery into the air. The camera controls were set to the DS' face buttons, freeing up the touch screen for radar and menu navigation. The touch screen is also used to hack into terminals and even other mechs, though the latter will actually hack back, forcing you to match numbers together faster than the other guy.
Phantom War will feature several multiplayer modes (local wireless and multi-cart), all in the traditional styles - king of the hill, deathmatch and the rest. It's not the prettiest game (even on the DS), nor was the demo exceptionally deep, but it could still offer up some decent action when it ships later this year.