Since all the scuttlebutt has been about the save situation, I figured I'd just throw this in this thread...
So, RE: The Mercenaries is actually a pretty decent game. If you enjoyed The Mercenaries in RE4 or 5, you'll enjoy this. Pretty simply, really. This is, however, a game that shows the hardware limitations of the 3DS, as Capcom tried to make it do things it obviously wasn't prepared for... the further away enemies are, the jerkier the motions of them become. It's a weird thing to describe, but you'll immediatly see it if you play the game.
In regards to the controversial save system... honestly, with the set-up of this, it really doesn't matter. The major things like characters are unlocked by the most simple progression possible, so the perks are really the only things that people may quibble at already having unlocked. Truth be told, I've put in a few hours already and never used them, though I suspect they'll be very helpful later in the game. Ultimately, it's not a big deal, and I honestly believe it was a design choice and not something to curb used sales. A used copy of this game won't really be 'gimped' at all. Now, if you're an acheivement whore, which is what the perks effectively are, you might miss unlocking those... but that's assuming that the person before you even bothered to go beyond the basic ones.
Content-wise, this is another PilotWings situation. If you pick this game up to simply plow through it, you're completely missing the point. I put a good three hours in last night simply taking EVERY character through the first 6 stages, usually repeatedly, because I wanted to hit SS rank for all of them. I do that for the rest of the game, I'm looking at 10 hours, easy... if not more. You can 'run through' the game with one character in probably an hour or two if you only care about progression... but that's just not what this game is about. Acheiving top ranks and playing with each character is the meat of this game, which people should know going in. Again, it's just like PilotWings... a very short game if you put the minimum amount of effort to progress into, but a very rewarding and long game if you're someone interested in doing as well as you possibly can.
The Revelations demo, by the way, is horrendously disappointing. We're talking about 5 minutes, and not even an exciting 5 minutes. You also can't change the options, so for someone like me who wants third person aiming and an inverted y-axis, it was annoying. Even if you have low expectations, lower them. It did, however, do the job of making me excited about the actual game. The demo, though, is awful.
Bottom line, if you enjoyed Mercenaries mode in anything else, this is worth a pick-up. Me, I actually never really got into Mercs on consoles, but I couldn't put this down. That said a lot for me right there. Not sure if I'd call this a full $40 purchase, but if you can snag it for any less (even $29.99), it's worth a look. I was okay with my $40, but I also got the free case from Capcom with it, so my cost to value ratio was skewed.