Buy it for the experience and gameplay, not the online features. This game is a must buy despite the terrible decisions made by From software on how the online play works. This game will scare you, trick you into dying and make you want to play more and more. There are a myriad of secrets and hidden paths for brave explorers and those seeking treasure to tread. The game will not give you an easy way through and will give those looking for a challenge a good time. Possibly with a little frustration along the way.
With that said, compared to Demon's souls using co-op or initiating an invasion (PKing someone for those who haven't played the series) is almost as difficult as this series is known for being.
You will often get "Summoning failed" when trying to initiate co-op by summoning a player who has put down their soul sign to try to help someone online. Additionally, if you are getting this game to play with your friends, count on a hard time even being in the same lobby/channel they are in. You cannot select your channel, so if you're not in the same one, then your friend could put down their sign all day and you would not be able to play with them. People who want to grief others by invading worlds and killing you will have a hard time also, as the level requirement to either play with or against someone is your 10% of your level. Meaning if someone was level 20, you would only be allowed to play with someone within levels 18 to 22 assuming they were on your channel.
The reason the online is so bad is that From has decided for some reason to switch from a server based online mode to a peer to peer infrastructure. I have seen alot of improvement with lag during online play, but there are still lagging players and there are barely any invasions compared to Demon's souls in which it happened fairly often.
That is not to say that the online play does not work at all. Co-op is still possible, you will just need to stand around for a few minutes and try summoning until the players are able to join your game. I heard changing your DMZ setting on your router for your PS3 may help with the failure rates. Still, players who have bought two copies of the game and are sitting next to each other may not be able to co-op due to the channels they are in.
Despite all this, I would say get the game. The online play works, just not very well at times, but the gameplay itself is something that will stick in your mind and give you a feeling of accomplishment. For those worried it would be a Demon's souls 2.0, don't be. The engine may be reused, but much of it is modified and the world is bigger, original and persistant. And did I mention it's also for Xbox 360 or PS3?
And here is one trailer that the game got that I thought was well done.