Sadly, it's very anemic for a game that must have sold about 700,000 copies, which is a shame because it's a great game for online multiplayer and it deserves to have far more players.  Apparently most of the buyers are single-player only people for various reasons.

(Note--all times are U.S. Eastern Standard Time, which I think is GMT -5)

I would say that the player population is about 70-80 people during European prime time (3 pm to 6 pm) and about 100 people during North American prime time (7 pm to Midnight).  After Midnight (U.S. EST) the player counts start dropping so that at about 3:00 am you'll probably find 50 people with the player counts dropping off to about 20-25 at 6:00 am.  It starts to climb steadily after about 10:00 am.  Player counts are slightly higher on the weekends.

Now, that's not as bad as it sounds if you play during European and/or North American evenings because you'll be able to find games and you'll probably want to play in the 5v5 pickup games (pug) matches anyway (where two team captains pick the other 8 players, resulting in balanced teams and intense, competitive games).  You will have to purchase the Entrenchment expansion for $10 (download with Impulse) if you want to play online since most of the regulars have it, but it won't bother you.

What you do need to consider is, what happens when Starcraft-2 comes out?  Will the player counts drop to levels where it will be difficult to find games during North American prime time, resulting in a further reduction in online player counts?  I think that the game will be fine for online multiplayer for the next couple of months, but it's hard to make projections after that.

On that note, let me reiterate that it's a great game for online multiplayer and you'll really enjoy it.  You should be aware that your average game runs for about 1.5-2 hours with most being decided around 1:15-1:30 and that a few games can last for over 2 hours with some epic games lasting longer.  (It's possible that game length has cut into the player counts some.)  I hope that you'll come join us.

Sadly, the online player count is small. But, the amount of players makes it much easier to form a community feel because you know everyone.

a large number of MP players use hamachi. I know I use it because its easier than trying to get ICO to work for me.

Wow 700,000 copies and yet only a few hundred every play online. I don't play online cause I don't have the time, nor do I want to, sure MP is probably the biggest attraction to which people buy a game. But I guess most people just have a life and they don't feel like playing 3 -4 hours a week online or they have a limited bandwidth space.