I wish more studios did demos so that they became more standardized and mainstream. Right now demos can be kind of iffy because I think a lot of companies don't really know how to pick and choose the best parts of a game... do they go with the beginning so that the user gets the full tutorial and is pulled into the game the way the developers intended, or do they throw them in mid-game after a few walls of tutorial screens brokenly teaching the user how to play? I tend to prefer the early game demos, but those don't always work when the game has a really complex control system because it can take a while to build up to a really fun battle system and you're missing out if the full thing isn't in the demo. And games like that, where there are a lot of nuances to the controls that take a while to master, are usually the kinds of games where a lot of people are on the fence about buying and would definitely want to try it out before committing, so I understand it's difficult to decide whether or not to make a demo and if the answer is yes, what to include. I think it's something that will be easier to peg down as demos become more common for all genres.
I like being able to check something out before I make a final decision on buying it. If a game I'm on the fence about has a demo, I will always download it and check it out to see if it's really something I want to get, or maybe something I just want to pick up later on down the road when the game has dropped a bit in price.
I don't understand the companies that make demos and then pull them once the game has been released. I mean... really? If the whole point is to whet peoples' appetites, wouldn't you still be doing that once the game has released at retail? People are always going to be discovering it, I can only imagine that if you were getting sales from the demo before the game went live, you'd still be getting sales from the demo afterwards...