I hadn't been sold on Halo for a long time, then bought an extra XBOX controller that came with a demo and was hooked. Got the sequals day 1.
I was a NUT for MGS, playing it straight through back-to-back one night to unlock everything. #2 rolled around, and I was all over it, but was dissapointed. I slept on #3, and when #4 came out I was actively nauseous just looking at the Raiden vs. Vamp trailer. Tried the demo and thought it was the most poorly made game I'd ever seen, and swore it off, only to be todl be everyone it was the second coming. I just can't get into it.
I was also non-plussed by the Uncharted demo, thinking "well, this will never sell..." That's not really quite the same thing, and I'll probably pick up Uncharted before the sequal comes out, but I have to say that Sony hurts itself with the demos it releases. Even the Killzone 2 demo looked drab and unimpressive.
On the Microsoft side, I've owned Gears of War since shortly after launch, and have played it twice. I also slept on Bioshock.
Believing in the PLAYSTATION®3......IS.......S_A_C_R_I_L_E_G_E
