Absolutely.

I was sold on The Force Unleashed after the demo

Ninja Gaiden Sigma's demo, and ridiculously high review scores, convinced me to buy the game (something I've always regretted)

I was going to get BioShock anyway, but the demo was so incredible that I had to buy it immediately.

Conversely:

The Enslaved demo left a bitter taste in my mouth, and I was expecting it to be brilliant.

The Mafia II demo convinced me to never buy it (I was considering doing so when it was cheap)

So yes, demos have made quite a difference for me.

ironically, I've downloaded more demos to my DS than I have to my PS3

When I don't have to play a demo to know godforsaken mafia II, Ninja Gaiden, Enslaved are crap games, I know I am an incredible person :P

But honestly, I dunno why people need to play the demos when there are reviews and gameplay (youtube) videos.

I mean honestly, just by reading 3 reviews and watching some gameplay of mafia 2 on youtube I knew it was shit. 

If you ask me, this generation of gamers are spoiled ... although there were a few demo discs back in the day, demos weren't as widely available as they were today. And despite that, great games that didn't have demos were selling a buttloadload while bad games avoided. Magazines and word of mouth were all gamers needed. 

I'm really against this ideology that "omg etc demo saved me a purchase because of etc etc" or "omg that demo was omg omg omg I'm going to buy this game now"

or maybe im anal because my flu symptoms are back and I'm grouchy at this hour.

I highly disagree with Ninja Gaiden being called a crap game, lots of people loved it/love it... it's more like how demon soul's is percieved (not in style obviously), but since it's somewhat difficult, some people love it, some people hate it.

Plus that game reviewed really well... so yeah.

I'm not saying you can't not like it... I'm just saying... that doesn't fit the categories of Mafia II... and enslaved isn't released yet.

^ yeah, crap is a pretty harsh word for Ninja  Gaiden.

If it makes you feel any better I was thinking solely of mafia 2 when I was writing that. Not saying that I like Ninja Gaiden though.

They can make a difference, although usually I have the right opinion before I get to try the game myself. Reviews and all bits of information just do that. That said, there are games I don't know much about and there are games that are so strange or different I need to try them myself. But the average shooter? I don't need the demo usually.

You have a fair point regarding reviews and youtube videos on games, but I don't understand why you would rather read and watch videos about these games rather than actually play them. Getting hands-on time with a game is one of the best ways to gauge what your feelings towards it will be like, even if with many games you can tell roughly what you're going to feel before playing it.

OT: Demoes are definitely important for me, if I'm on the tipping point on whether or not to get a game, demoes can be a great help, and some times I try a demo of a game I hadn't payed attention to at all, and then I just have to have it.

I was interested in Valkyria Chronicles, but didn't really know whether to get it or not before I tried the demo. The gameplay was just awesome, and I saw a lot of potential for more awesome in there.

I hadn't payed any attention to DeathSpank at all before it released, but then I tried the demo and I had to own it immediatly. Awesome game!

When I first got my PS3, I downloaded the Burnout Paradise demo, which also made me 100% sure that I had to own the game, even though I hadn't payed any attention to it before.

And I was excited about Batman: Arkham Asylum, but not sure if it would be good, and the was one of the greatest demoes I have ever played. I had to own that game, no question.

Those are the cases I can remember at least. 

Demos mostly make me not to buy a game, happened

But with games like Kameo, Dead Rising, Enemy Territory: Quake Wars the demo was what tipped me over to actually purchase the game.

In general I wish there wasn't any demos though. I want to buy games completely fresh even if it means taking a risk.

Oh, that Crackdown 2 demo......good lord, Microsoft.  Why did you subject me to this?  But, like others have said, most of the time, I already know what I'm going to buy because I do a fair amount of research.  Usually, if I play a demo, it's because I'm on the fence or I'm really bored.  Every now and then, I get pleasantly surprised.  There are some demos that will never get a chance because the game never interested me in the first place.

--Would a game like Halo or Gears sell more with a demo?   Again, like someone said, some of the best games sell bucketloads without a demo.  Are demos obsolete?  Back in the days of the PS1 and PS2, I was a proud memeber of the Playstation Underground!  I loved getting demos.  I wouldn't stop until I played everything on the disc and watched all of the videos at least once (I played enough Grind Session and Crash Team Racing demos to kill a normal man).  As time progressed, I played less and less of the content on the disc. 

Burnout: The demo got me to buy it right away and for 85$. Was planning to get it later on. Today I feel that it was worth every penny.

Fight Night Round 4. Easily my favorite demo. Played it more than a number of full games. Also loved when they made a new demo that lets you play 10 rounds and has Ali and Tyson. Though I kinda lost interest in the full game because of that. Still play the demo a lot, though.

InFamous: See burnout but for 50$ off a friend. Then 30$ from the PS Store.

Naruto (probably the most interesting one): My bro and I LOVED EVERYTHING about the demo. We both got our copies ASAP. That was after we spent easily atleast 15 hours playing it. However, I ended up hating the game. We both just downloaded saves that gave us all characters. Also, free battle wasn't as fun as the demo. There were glitches in the demo that actually made it more fun and technical than the actual game. I think he still has his game but he redownloaded the demo. While the game was fun I ended up selling mine but redownloaded the demo a little while after to try it out again. Don't have it on my HDD anymore, though.

Quantum Theory: Wanted it... after that demo.... not anymore.

Resident Evil 5: The demo got my bro and I to buy not one but TWO copies for 200$ (together) right away. Co-Op was just that fun.

Uncharted Series: Had no interest in Uncharted1. The demo got me interested in the game though and along with it's multiplayer demo, got me to buy part 2. Enjoyed part 2 but felt it was overhyped. For some reason while the demo was great, Uncharted 2's actual multiplayer didn't really feel fulfilling for me for some reason. Heard there's a number of people that felt the first was better than the second; small minority but I'm hoping when I do finally get the first I fall into that minority.

Valkyria Chronicles: Demo gave me a very good impression and I had already wanted the game to begin with. It's also what brought the game to my bro's attention and he really wanted it to. After I actually got it, it was fun initially but I lost interest after about 9 hours. Story wasn't very interesting. Certain things pissed me off. Felt like I was waiting a lot longer than nessecery for things to happen. Lent it to my bro, he finished it and felt it was good. Lent it to one of my buddies, he loves it. Got stuck near the end at a boss though. As for me? Guess I'll never finish it.

Vanquish: Planned to buy when it's around 30. Now I'm thinking to just rent it some day... eventually.

That's all I can think of right now.