I've seen the HC7800, -7900, -8000, and -9000D, but not the -5. I purchased the HC9000D after the -7900 and -8000 came out, so I made a deliberate purchase for what I perceived as the best one.
I agree with gbickle that the HC7800 should be avoided. It had a lot of promise, but the implementation was weak. As for the HC7900, it's a bit better, but I just don't like having a white projector sticking out like a sore thumb in my HT. The HC8000 is a better all around performer than either of them.
Where our opinions will diverge is in the final recommendation, which for me was HC9000D all the way. At this juncture. some guys would start throwing around hyperbole like "X just totally blew Y away!!" The HC9000D didn't blow the HC8000 away; it was just better, in some cases much better, than the HC8000. I'll give three examples.
- As gbickle already mentioned, motion blur was an annoyance for me on the HC8000.
- Rainbows are sometimes a problem for me, and a couple of times I seemed to catch a sparkle out of the corner of my eye. Real or imagined, it's still a distracting problem. No rainbows ever on the HC9000D for obvious reasons.
- The colors, contrast, and film-like quality of the HC9000D were very impressive right out of the box and were amazing after calibration. The HC800 appeared somewhat sharper, but not in a way that I appreciated. Perhaps I was lucky, but the sharpness on the demo HC9000D I saw was just great, and so is the unit I bought.
Having said that, I would have gotten the HC8000 with little reservation if the HC9000D were not still available at the time and the HC5 had not yet come out. I think it's a solid choice.