Just played a round. You guys aren't really missing much...
There's not even a benefit to having it 'early', really (I expected you'd have limited access to unlockables, but that's not the case).
It's a pretty generic deathmatch demo (only saw one level, not sure if there are more). It was a ruined temple area with a Domination game type (Data Post Control, other mode available being Elimination). Not much in terms of guns, but this could just be limited to the level.
And when you finish the round, it says 'Coming Soon' and kicks you back to the main menu.
I'm not really complaining, but I just figured there'd be more to it than that. You don't keep any EXP that you gain in matches, and you can only (slightly) customize your weapon loadout [not entirely sure what this does, as I was limited to a machine gun the entire time, but I did see someone pop out a shield].
Bring on the full game already!