Personally I like demos.

They let me test drive a game before shelling out $50 or $60 bucks.

In some cases it's saved me from wasting my money. I've seen games get great reviews that I ended up hating and I've had games that I love get mediocre reviews.

Renting games is just about useless they days.

New games are never in and by the time you can rent them, 75% of them are already trashed.