It's not like the electric plasmid the entire time, where you "zap 'em and wack 'em, the ol' one two punch, THE ONE TWO PUNCH". You get better guns along with the ability to upgrade them, and a variety of plasmids as well. There is alot of variety in how to kill people. But CoD this is not, There are no short well defined chapters, or hectic war scenes.

The demo pretty much perfectly shows the gameplay portion, minus the extra weapons and plasmids.  Depending on how you build your character things change, but every encounter in the game is essentially the same because no encounters are actually setup (except for a few specific ones). 

They tried to make Rapture a living world where the AI just wanders so it's always going to be run around, find a splicer, kill them the way you know how, and then continue walking around.

If you don't like that gameplay you won't like Bioshock.

And even better than fighting one Big Daddy, is having two of them fight each other.  There's nothing like getting some easy Adam at virtually no risk to yourself.

How long is the demo, roughly in playtime?

If it's the same as the 360 one, probably a good 45 minutes on your first playthrough--everyone owes it to themselves to at least play the demo and try the game for themselves.

If the demo didn't do it for you, I doubt the improvements throughout the game will be enough to keep you hooked until the end.

The game is severely overrated, let me tell you. As soon as you get the electric gel and crossbow no enemies are a challenge at all and it gets boring. The infamous and "dangerous" Big Daddy's sport piss poor AI and are only tough in the beginning when you have a wrench and a pistol, money is not an issue and you can buy heaps of AP rounds to blast them to hell while getting them stuck in many obstacles. Other enemies are equally stupid, running headlong at you, leaving you ample room and time to shoot their heads off. The bosses are very hard, making the game as a whole very unbalanced and frustrating.
You have also probably met every variation of enemy in the demo alone and seen most of the environments the game offers as well as tapped into most of the gameplay elements.
The plasmids are arbitrary only, like the suit in Crysis, and is mostly a gameplay gimick justified solely by the few instances where you HAVE TO use them in order to advance in the game.
A clear weakness in game design if you ask me, and I could easily play through the entire game without ever needing the special powers.
The revolution was not there, it is a good FPS, make no mistake, but don't expect a vastly superior shooter that offers much outside of the strict perimeters this genre has set itself.
Also; the story is kinda cool but told in an extremely idiotic and non-interactive way.

Long story short; SFX is amazing, the best I've heard in a game, the atmosphere is very good and the visuals are very good. The gameplay, the story and the narration is very, very shallow and the game bored me to the extent of making me quit maybe 2/3 of the way (unlike Crysis, that made me quit halfway).
Bioshock was to me the second biggest letdown of last year (the above should help you figure out which took number one).

I did not like the demo on the 360 but played and beat the game its worth buying if you havent played it

I wasn't going to get Bioshock, but after playing the demo for 15 minutes I'm starting to reconsider. I've checked out a few gameplay videos; it looks like the real deal.

if the ps3 demos is the same than were in the 360 of course it does... its only the first 20 min of 15-20 hours fo the game. Just for comment, i prefer Bioshock on first place, half life 2 second, and metroid prime on third.