I'm sorry, but I disagree with both Reggie and other people who are convinced that a good game will definitely sell well. There are tons of other factors that need to be taken into account.
Online multiplayer games tend to sell very well especially at launch because of all the hype. Several of my friends felt forced to buy the next COd game just because their friends did and they wanted to play with them. Same with Battlefield. Also, when several multiplayer games tend to lose some of their audience, so gamers feel like they have to buy the game at launch, i.e. new so that they can enjoy a good online community.
Single player games on the other hand tend to do much worse and are bought used again and again. If you can beat god of war 3 in two days, then why not trade it in and get some money back? If you ask one of the vgc members who's a gamestop manager, he'll tell you just how many used copies of God of war 3 they received. With the online games, I peresonally never feel that I'm done with the game, so there is no need to sell it back.
I have been nothing but impressed by the new DMC game, but I still haven't bought and I'm waiting for it to be around £10, because I'll enjoy it just as much whether I buy it now or a year later (several people who played it, including reviewers, said it was pretty awesome, so its sales don't reflect its quality). In contrast, I bought Sould Sacrifice at launch to make sure that I could team up with other people and thus payed full price for it.
I bought wiifit and hated it and I doubt that its sales reflect its quality. Some times it's mostly hype rather than a good game. If sales reflected quality then Xenoblade would have sold much better than it did. Didn't FFXIII-2 outsell it despite bad word of mouth from XIII and average reviews? Was Xenoblade a bad game or what?
