Demos, trails and betas make up nearly 25-30% of my gaming time really. I love to test out new things, I love to see what the upcoming games will look like and it let's me actually see if there is anything interesting that might have slipped under the radar.
Often times I will even play demos/previews of games I'm not interested in, just to see what are they about and if by chance they have some ideas/mechanics that may be nice addition to other games.
Guess my love for demos comes from good old times, when games were rare, far between and especially in my country, pain to buy, while many of the monthly magazines included 1-2 CDs packed full of various demos I could play one after another.
Sadly in recent years demos either don't come at all, come post release or are just very poorly done which pains me a lot. I generally don't buy games at release unless I'm sure I'll enjoy them, so the less the dev/publisher let's me see before the product hits the shelves the lower the chance I will give them my money.