I love the idea of playing before release. For me, a demo/BETA has always put me off or on to the game I'm testing. BETA testing the new Medal of Honor game, I hated it and will definately need to test the demo in single-player before I try the real game. The same went for Il-2; the demo put me onto the game. I doubt I'd have bought the game if I hadn't tried the demo, as I had seen the game in shops and thought it would be some really bad Russian version of CFS1
How wrong I was.
Anyway, going back to the idea of a BETA, I have always liked the idea, but then again I only buy a game once I've read some reviews, seen in-game pics (of the released game) and maybe watched videos on youtube. I've been stung buying games before seeing videos and reading reviews, and I don't want to be stung again.
In this case, I wouldn't buy the game online to just test the BETA.