Well I would just wait until the online MP comes out. It's only a couple of weeks away. No harm in waiting a couple of weeks is there?

You can definitely still have a fun time in singleplayer, it's still competent.  It's definitely designed for coop but they haven't made it impossible or unplayable in singleplayer.  Basically, if the idea of the game appeals to you - get it for the sp and then you can pick it back up in September when the online coop comes!

I'd say yes. Single player is fun and it has some replay value to get all the items. If you like the demo, go for it :)

Yea I'm really liking it.  A lot of depth to it and the combat is really satisfying.

I've only played it co-op so far. About 4 hours. But after that experience, I can't wait to go back and play through it on single player. So while, it may be a bit better cooperatively, I would say its worth the money. The coop will come, and its surprisingly deep and rich for only $15.

I've been playing both singe player by myself and multiplayer with my girlfriend. It's quite a different experience because you need to tackle most of the puzzles in a different way. I'd say it's definitely worth it for the single player, and different enough in co-op that you'll want to play through it again in a month when you can play online.

While the single player is fun if you manage to get a lot of the challenges & main story done it will take you about 5-6 hours roughly depending on how stubborn you are with trying to get some of the challenges. After getting the game today I definitely plan on going back to get better though it clearly isn't as replay addictive to me as Shadow Complex, highly recommended.