I've said this about 10000 times now: there are many people with ridiculously inflated notions of the appeal of Metal Gear Solid.
It's kind of like going to a Star Trek convention: when you're there, it seems like Star Trek is incredibly popular and everyone loves it, but then you go back to reality and Star Trek shows haven't made good ratings in years and practically no one cares about them anymore, in a larger sense. There are still a lot of people who care about MGS, but the same overestimation of fan base applies.
In other words, it's hard to keep Metal Gear in perspective, because the internet forum crowd is so rabid about this franchise. But it's not nearly as big as Halo, and recent comparisons to GTA (lol) should be laughed out of court. It's a big game. A lot of people like it. We should leave it at that. I expect it to sell about as well as Mario Party 8, which would be an improvement for the series and would show that the fanbase is still around.
You always underestimate MGS. The comparisons to GTA were PS3 version only.