This game is amazing. It is definately playable and plattable offline, but you will be missing some of the most interesting/enjoyable parts. I know everyone, including myself, freaks out at the thought and end dates of the servers, but if they are selling greatest hits versions i really doubt they will kill the servers next year.
Playing online is fantastic because if you ever get stuck there will be notes to read, spirits to follow, blue souls to help you out(if your in body form), and if you are in spirit form you can help other bodies and get your body back while practicing on the bosses. I found it fantastic. The last times I played it was so long ago, so this is just reminiscent good thoughts, but i loved the mood and everything about it. I ended playing it because I got caught up being a red invading spirit and I kept losing my levels cuz i would fall off the stage or get killed by the environment and it drops you a level, lol. at level 120ish thats a ton of souls and i was like, i gotta knock it off. but im totally gonna go back someday, hopefully while servers are live.
oh word of advice tho, i know people say it is very hard, but whatever you do, don't just summon blue spirits and follow them through the levels. it will make the game 100x easier but alot less fun because the difficulty is important. i spent a level or two summoning blue spirits and just letting them kill everything cuz i was freakn out but then it makes it too easy. i know im crazy for thinking demons souls is too easy, but its true if you have a blue spirit army with ya... yow
oh and world 3-3 fight is epic. and so is 3-1 cthulu prison guards with chime bells...oooh it makes me want to play it so bad when i get home from work! oh yeah i got the collectors edition too for $40 the day it came out, woot, i always kick myself for not tearing off the strategy guide cover and requesting a new one tho, oh well.