If the game is good, I will play it. If the game is for sale, I will pay for it when required.
My thing is, I would rather have a completed game to play when I start it, not some beta or even a demo. I will do some demos as the first 30 minutes are key to getting me to like a game. However, I would still rather wait until the game is ready to try it out. It is not whether the story hooks me or not, but does the game play hook me or not. A great story in a crappy game will get left on the shelf. Great game play can over come any story defects, or any story at all.
Hmm, exact opposite here, Great gameplay does not excuse a poor story for me, but I'll excuse terrible gameplay if the story is fantastic ^^
As far as the alpha/beta bit goes, the few times I've done betawork I've stuck around for a good while in the finished product, mainly because I've firmly lodged myself into the core fanbase, and am generally a mover and shaker in that particular game and I rather enjoy that status ^^ I'll keep playing a game for quite a while if that's the case, even if I'm not particularly getting any enjoyment out of the game itself anymore. Examples(not limited to just betas) are FFXI, where I played for several years longer then I really felt like because I was a core player on my server, knew just about everyone, and was always doing something with someone. Or Utopia! A browser based, text-only medieval fantasy war game. I started playing that thing in the alpha, and stuck around for nearly a decade past that, many years after I had already mastered the game, because I had formed and led a kingdom of people I knew to the top spot on the server, and I wanted to stick around for them. It's sister game, earth2025 I did the same thing with as well.