The LN-T4066F has a newer main board (1009) and more recent firmware. This helps avoid some intermittent green and pink color "flashing" I've heard of from 4061 and 4065 owners. The 66 is also rumored to have deeper blacks than the other "406X" models, although I haven't seen all of them lined up and properly calibrated for optimal black levels so I can't personally verify that. I can verify that the dynamic contrast ratio on the 4066 is 15,000:1 versus 10,000:1 for the 4061. The 4061 and 4065 models are still strong sets and should be less expensive than the 4066 at full retail, but there are additional considerations with those.
After I have the TV a while longer, I'll probably write up an Epinions.com review on the set as most of the user reviews available online for the 4066 aren't very detailed. Some things I like other than the PQ:
1. All the buttons are heat sensitive (like the PS3).
2. The remote is intuitive, even if the buttons are a little small.
3. Swivel base!
4. 3x HDMI inputs.
5. Dual-source Picture-in-Picture. Yes, you can watch a Blu Ray movie on your PS3 while checking ESPN on the smaller PIP display at the same time. And yes, you can swap audio between the primary and secondary displays.
6. It has a setup feature that will automatically identify A/V sources attached.
7. Single-touch input source swapping.
8. Will display digital pictures and play MP3 music via "Wiselink" USB input from thumb or flash drive. Biggest MP3 player I've ever seen!
9. Appears to support 24 frames-per-second video signals for better HD resolution.
1. The TV supports audio output via "RCA" cables, but not video output. Won't make a difference to most, but does to me as I have an Archos 504 portable DVR that can't input upstairs from my dish receiver (it's downstairs).
2. Bright backlight helps, but reflective screen can be an issue for rooms with high amounts of ambient light.
3. My universal remote appears unable to learn "Info" button functions from included remote. Might be a "me" issue of course.
The Sony Bravia XBR LCD series, as RFS noted, is another great line with excellent picture quality and detail. If I didn't get such a great deal on the Samsung set I purchased, I'd still be on the fence between that and a 40" XBR LCD. I have to admit that the Sony XBR series is nice looking, but as my setup (PS3, Sony surround sound, TV stand) is all black, the Samsung is damn sexy.