First impressions, upgrading from the HD1000u to Optoma HD20. This will focus on the differences I observed, whereas I will post my main review in the official HD 20 thread. Caveat is that my theatre is quite modest - projecting 145 diagonal on a bare white wall, in room with no light control (wait until dark to watch!), white ceiling and very light carpet. Seating distance about 14 feet from screen.
Start up time - HD20 start-up time is significantly faster, and when operating lamp in the low mode, does not step down from high like the HD1000u.
Fan noise - I'm pretty sure the HD20 is quieter in both mode, despite what the dB specs. say.
Optics - perhaps a little better than my HD1000u, since the color fringing on a black and white test pattern is less than with the HD1000u - not that this was a real issue with the Mitsu, however. No signs of uneven focus; across the entire field can see the pixel structure OK.
Light spill - partly since the HD20 cabinet is white, the light spill out the front looks much more prominent, though in this environment (of poor light control) the degree of light spill has no discernible effect on the image.
Shut down time - significantly shorter than the HD1000u, and the fan speed goes WAY up, so is very noisy for about 30 seconds. First time this happened it scared me as I thought maybe that would be the sound for operating lamp on the bright mode (as in the HD1000u), but thankfully it is not. Looks like they just want to do a turbo cool on the hot lamp.
Picture quality (drum roll) - not a dramatic change from the HD1000u when watching HD content from cable, but a more subtle effect, as in a smoother, film-like image due to the smaller pixel sizes. I used the test pattern recorded from HDnet to do some tweaking and evaluation. Initially I thought the HD20 image had lost resolution versus the HD1000u, until I examined the resolution test pattern and realized that dithering or Nyquist frequency effects on the 720p HD1000u make the resolution bars merge in an irregular pattern, whereas on the HD20 there is a smooth transition from the separate bars in the wedge to a smooth blur. This phenomenon undoubtedly makes the images more film-like, and echoes the comments by projection CRT enthusiasts that "digital projectors display harsh edges that are not natural". By the way, I am only getting around 700 horizontal lines on my HDNet test pattern (for either projector), so wonder if my cable company is down-rezzing the channel - has anyone else observed this effect? Shouldn't I be getting >1000 lines since there would in theory be 1920 pixels across the image? Final comment is that I briefly hooked up blu-ray player, and it automatically fed the HD20 an 1080p 24fps signal through HDMI, and the image quality was extremely good - which suggests that my source feed (from cable DVR) is probably so crappy that it doesn't matter whether the PJ can display 720p or 1080p. A proper image test would be to run a blu-ray test pattern disk.
Bottom line - a nice, incremental upgrade from the HD1000u, but if you're happy with your HD1000u, you shouldn't feel like you're missing out on a hidden world by postponing an upgrade to the HD20. This is my opinion formulated after 3 hours of exposure, so keep that in mind, too, and await the professional test reviews get some hard facts.