As topic - is it possible to play this game as a purely single player experience, without having other people "invade" your game or otherwise enter it? I've found this information maddeningly difficult to come by in reviews, etc, but am very interested in the game, because it sort of looks like a Severance - Blade of Darkness kind of thing, which is rare these days.

Yes, it's possible. Just create an offline Windows Live profile, and you won't ever have to bother with other people.
I know after starting with an online profile I was really annoyed by a few assholes who camped the entrance to boss rooms to force pvp on me, so I went with an offline profile.

You lose out on some of the experience, and it can be a bit harder since you can't recruit help for parts of the game where you have a harder time than usual, but imo it's worth it if you dislike pvp like me.

Thanks for the quick answer guys. WIll be picking this one up soon. Out of curiosity, does it resemble Severance Blade of Darkness in gameplay at all (for anyone who happens to have played both?)

When I was telling a friend about Demons Souls I used Severance as a reference point.

Severance is level based but Dark Souls is more open world. Combat is similar type style, block and attack, you have to watch your stamina also.

The only bit you will have difficulty with I think on Dark Souls is the twin bosses, Ornstein & Smough. You can summon a NPC to help you out, but I also summoned 2 human players to get through the fight. Managed the rest though.

Only major issue with the invasion side of things on the PC (360 also) version is the cheaters. They max out their characters stats but keep a normal low type level so they can invade and never get beaten. I have it on PC but thought console was better.

I think I only got invaded about 10 times or so my entire 90+ hour play through on the PS3 version. 1 invasion was really cool though, it was like a game of cat and mouse and we had both run out of health flasks, arrows, magic and everything after about a half hour. I died as I got hit by an exploding environmental floating skull enemy. Few minutes later I got a message from the guy that invaded me saying it was an awesome battle and a joy. Cases like that though are few and far between.

I would highly recommend using a gamepad while playing also, control is better suited that way.

Offline GFWL is the only way to GFWL.

Alternatively, if you want to keep things like messages and ghosts, you can just stay in Hollow form. That way you won't get invaded or most of the multiplayer experience, just passive communication.