I'm only interested in the GoW Collection, GoW III, Demon's Souls, and the upcoming Last Guardian. 
 
Is it worth it for just these games?

Well you'll be getting a blu-ray player and I think you'll end up buying more games if you have one. There is great exclusives on the PS3, especially this year.

" Well you'll be getting a blu-ray player and I think you'll end up buying more games if you have one. There is great exclusives on the PS3, especially this year. "

what's with people posting "should i buy x" threads?
 
nobody can answer that question but yourself. if you want to play those games, go for it. if not, don't.

No. God of War III is a terrible game, and you'd have a far better time watching a playthrough on Youtube than actually playing it yourself. Demon's Souls is great, but not great enough to buy a console for. The Last Guardian hasn't been released yet and they've barely shown anything of it, so it's hard to comment on that, but if the developer's track record is any indication, it's going to be really beautiful, but also really shallow.

You really have no interest in any other Playstation 3 exclusives?

I bought one, then sold it about 8 months later. I hated pretty much everything on it. Just my personal experience.

I mostly agree with @Icemael about God of War III. The game looks awesome, but is CRAZY cheap and frustrating at times.  
 
That said, I got a PS3 almost a year ago and haven't completely regretted it. I like MAG, Fat Princess, inFamous... If you have the money for both, just get it. Just be aware that if you have a 360 you'll still probably spend more time on it due to the better online integration.  

I too was thinking about buying a PS3 someday, solely for a few exclusives. The only games I really want is LBP 1 and 2, ModNation Racers and Demon's Souls. But I'll wait for some kind of super sale on it, since buying an expensive console for that few games is kinda silly. So I guess that if you need a Blu-Ray player, and you have the money to buy one for cheap... why not?

The system has god damn Uncharted 1&2.
 
Go buy it.

Is it worth it for just these games?

buy it and you also get a bluray player in it. bluray is what to get now instead of DVD. there has to be other games that you want instead of just those games.

Dude do whatever you want with your money if you wanna get it, get it, maybe you'll later on get into the uncharted series or another good exclusive

Buy an Atari 2600. They're dope.

Forget God of War 3, and get Ninja Gaiden Sigma 2 instead.

I'd be tempted to pick one up if I had the spare cash, there are a few PS3 exclusives that I'd really love to spend some time with. Uncharted 2, little big planet 2, demon souls...

The God of War games are definitely not games you should buy a system for. Look at what Icemael said, that pretty much sums it up. Demon's Souls is a great game but it's a very niche game that you may not even like. It's very love/hate. I lent it to 3 of my friends and none of them liked it, and these are guys who I thought would be head over heels for it. 
 
The Last Guardian - I'd say just wait until this game is out to see if you really want a PS3.

I think it behooves one to have both a 360 and PS3, though the vast majority of stuff is for both. 
Since this console generation is probably gonna last for quite a bit longer, if you can afford it, I would say owning both is worth it.  

It's worth it for just Demon's Souls in my opinion.  Either way, you should buy one.  I did last summer and it is the best investment I've made since buying a PS2! 

There's also this: 
 
  

First things I played since I got my PS3 were the GoW 1-3 and Demon's Souls, I thoroughly enjoyed my time with all of them.

Yes.
 
There are dozens of amazing games that you can't play on any system, and if there's only a handful of those that interest you, well, you're missing out big time.

Great time to buy a PS3 I think, even if you are only looking to play those games. Doesn't mean you'll never try out some of the top titles available on the system.  
 
I'm actually pretty damn excited about what the PS3 as to offer this year. Certainly more then Xbox 360. 

Worth it for Demon's Souls. 
 
There are tons of other great games on PS3 though...

" Worth it for Demon's Souls "