While not a robust demo, it definitely showed off the potential of the full game.
Resident Evil: The Mercenaries 3D, which is set for a summer 2011 release, is a compilation of the Mercenaries secondary mode present in Resident Evil 4 and 5. Featuring characters and settings from all over the Resident Evil series, Mercenaries 3D also serves as a celebration of Resident Evil's 15th anniversary. While we can't confirm that this game is going to be packed to the gills with content, what Capcom has told us so far is very promising, and the gameplay in the two-level, four-character demo is rock solid.
The demo has four playable characters: Chris Redfield, Claire Redfield, Hunk, and Jack Krauser. Each character has a different loadout with different accessible weapons and items. My personal favorite weapon in the demo was Claire's electric-based weapon that shot out a pulse that hit multiple enemies in a certain area. Jack Krauser, naturally, is a total bad-ass with his bow and knife.

In the demo, there are two levels, one each from Resident Evil 4 and 5. If you're not familiar with the Mercenaries gameplay style, then what you need to know is that it is a score-based survival mode where you have to last as long as you can with limited supplies and an endless onslaught of enemies. Basically, it's Horde mode before there was a Horde mode.
You control your character with the Slide Pad, and activating your aiming reticule by holding the R button. Then, you can shoot with the Y button. Like Resident Evil: Revelations, you can hold down the L and R buttons to lock your aim and walk forward or backward. Changing weapons and using items is easily done on the touch screen; no need to worry about a stylus because the on-screen buttons are big enough to just hit with your finger.
After a little bit of getting used to them, I loved the controls. I found myself generally holding down L and R to lock my aiming, though, otherwise I would usually mess up my aiming because I'd be moving while trying to fire.

The 3D effects are subtle, but most definitely immerse you in the experience. Add in a pair of headphones, and it is an intense game, especially when the enemies pile on top of you. Is the 3D necessary? No, but it makes the game better.
If Capcom delivers on their promise of a bunch of content, Resident Evil: The Mercenaries 3D will be an awesome single-player and multiplayer (local and online) game. This demo rekindled my love for the mode, and I'm very excited to play more of it in 3D.