It says it in the TITLE
The game does exactly what the title says. It throws you into the mix of Mercenary mode of RE4 and RE5, more of RE5 though. Yes, there is no main story, we know.
There are several characters to play as and each one has their own weapon set, finishing move, a couple of melee move and an additional costume. The characters look kinda cartoonish, colorful and flamboyant in the select menu but they look pretty realistic and crisp when you play.
There are also about 30 different perks that help you advance. Most perks themselves have 3 levels of upgrades. The level goes up as you play and as you use their benefits. You only get 3 slots for those perks though and you can change them in between characters and maps. Some of them are pretty much useless though. And there are 50 medals or modern day "achievements/trophies" to unlock.
There's lot of replay value because you can play as various characters on different maps and attempt to achieve those "S" or "SS" ranks. There are also a few boss battles. However, this replay value can only last for so long. There's no difficulty setting and most perks would take a long time to unlock. So somewhere in between 7-8 hours of playing you will pretty much grind a halt, perk-wise and new costume-wise. What the game does is it give you the ability to unlock all characters before you even finish all stages and it gives you about 5-6 perks easily. Then the game throws you back into a stale state of replaying the same stages hoping that new perk would unlock.
Anyways, the graphics are pretty good, the maps are slightly changed from the original versions and the gameplay is actually as fluid if not more fluid than in the original version. True, you have no right analog stick (yet) but at least you can move and reload at the same time. And trust me, that little change makes you extremely mobile. The aiming is slightly harder to pull off here though only when it comes to head shots. But the game does not reward you for head shots as much as the original versions because you'll have enough ammo most of the time and you simply wouldn't waste a lot of time aiming for heads.
When you zoom in with a rifle you can see the well distinguished facial grimaces of the zombies. But When you see the zombies run from afar at you, then you can notice that they kind of move like stick figures, as if they are not completely synced with the screen. It looks like they vibrate at a different frequency than the rest of the world, so that is one of the minuses.
I personally used Type 4 gamepad mapping because it represents RE4 main mapping. You can choose 1-4 types of mapping in the option menu.
There's also an option to have a first person view while aiming and it sucks, plain and simple. The screen is already small enough and to have an even narrower view is suffocating, literally, stick with the original formula.
The demo for the Revelations title only lasts about 4-5 minutes.
All in all, this is a pretty good game but it's best played with a near-by friend who has the same system. However, if you are to play the game on a 3DS at home, then why not play RE5 on a PC or a console instead?