I use my 12k with a 106" Dalite High Power screen. The projector is set up about 18" over viewing distance when seated so I am getting a lot of gain. I get a picture that is plenty bright in High Contrast/economy lamp mode (the iris closed as far as it can be and the lamp on economy setting). I would guess that if I did not have a high power screen that the picture would be too dim to view in High Contrast mode. BobL is probably right about his screen size recomendations if your screen does not have a lot of gain. The alternative is to open the iris or use the brighter lamp setting.
My projector is set up about a foot behind the seating area and 18 inches above. It is not the quietest projector out there. Fan volume is probably this projector's weakest link but it does not bother me under normal viewing conditions, even when in the seating location closest to the screen. Others will probably not agree.
One thing to note is that the projector ships with colors at the 8500k setting. Don't keep this setting. Changing them to 6500k is as easy as pressing a few buttons on the remote. I don't think that the 6500k setting is perfect but it is very close and not at all objectionable. There are no lime greens or anything like that. I also use the dynamic contrast setting, which emphasizes black detail.
If I had to sum up the character of the image that the 12k projects, I would have to call it film like, although that term is used too often. The high ANSi contrast ensures that images leap off the screen. Mixed light and dark scenes are particularly breathtaking because blacks are inky while whites are bright enough to cause you to blink a few times to allow your eyes to adjust.
I have now seen quite a few 1080p projectors (not the JVC yet) and still feel that the 12k is better than most. The only one that I have seen that is better is the Sharp 20k. Even then, although the 20k is spec'd to have much higher contrast, in practice, the difference is small and hard to notice and not worth an over $5k difference in price.