I bought an HD640 a few weeks ago. Its my first projector and I'm quite pleased with it so far. The picture quality and brightness are great.
Although, I have had the same flickering problem that romeojitsu had. When in STD brightness mode the picture flickers for 2-20min every hour or so. I tried running in bright mode for a couple days, as was suggested in this thread. I've seen no flicker so far in bright mode, but it still shows up in STD mode.
Should I exchange my projector for another unit? I would consider this a defect, but I wonder if a new unit will just have the same problem?