Organic panels have a much higher chance of showing "image retention" (similar to screen burn-in on plasma TVs). They also have a higher failure rate due to panel degradation. Due to design differences, the organic panels also may have slightly poorer black levels.
Epson uses them on their 6100, to keep the price down (and to use up their stock of old organic D6 panels).
As for the brightness issue, that is something only you can decide. It depends entirely on your viewing environment (and screen) and personal preferences. With my 106" high gain (= 2.8) screen and my preferences, the HC5500 is more than bright enough in the low lamp mode (even with a low level of ambient room light). With the drapes pulled, it's still bright enough (for me) to watch in the daytime, without having to switch to the high lamp mode -- but my room is on the NE corner of my house, and has only 2 windows.