I posted my impressions in the TK800M thread but I've copied it below if anyone is on the fence between the TK800M and HT3550. Scottyroo's review pretty much hit the nail on the head -- the TK800M is great in a living room environment with ambient light if you're looking for a versatile projector for movies, gaming, and especially sports. The HT3550 is much better in a DARK, light-controlled environment (I found that even the slightest ambient light would cut through the image) and has better blacks for movies. I ended up going with the TK800M.
I had a terrible time with the HT3550 -- my model was most likely defective. The colors were awful and no amount of tweaking could fix the colors to look right. I also had issues where the screen would be darker on the right half of the image and it would stay that way until I restarted the projector. There were times when the projector booted up and the image was all white with random splotches of black (see attached images). There were also sharpness issues and I couldn't focus the lens to make the image sharp enough. Lastly, since this in my living room, even though I have blackout curtains, if there's any sort of ambient light, then the HT3550 will not be bright enough. I found myself trying to make the HT3550 with WCG off to get the extra brightness, but it still wasn't bright enough. In the end, it wasn't worth the hassle of constantly trying to adjust the image or fix the various bugs that are present with the current production model. I bought on April 26th hoping that most of the early issues would have been gone, but my projector still had all the reported issues.
The TK800M was much better for my use case. It's a great all-around projector that displays a bright, SHARP image that works great for movies, gaming and especially sports. The blacks aren't nearly as nice and the HDR mapping HT3550 is much better, however, unless you have the right light control (which you most likely can't in a living room), then TK800M will be a better fit for most people. The only downside is that it has a shorter zoom and no lens shift which makes placement a little more difficult. I was only able to project a 99" image due to the distance restrictions I have but that trade off is worth it for the versatility, brightness, and sharpness the TK800M has to offer. I was also able to get the TK800M for 20% off at Fry's making my total cost $1122 including tax vs $1619 for the HT3550 from Benq Direct. At a $500 difference, the decision was a no-brainer!