I initially bought the LNT5271 and it had some board problems which did not preclude me from tweaking and viewing it for a couple of weeks. During the time I was waiting for a repair I considered a number of alternatives and wound up buying the FPT5084 plasma. I used both sets primarily for watching a large collection of SD DVDs that I inherited as well as SD and HD TV cable feeds. A few observations -
1. SD TV is not acceptable on the LCD, good on the plasma. There are exceptions, but the plasma deals with the issues of SD TV much better.
2. HD TV is good on both sets.
3. Still and slow moving picures on the LCD are fantastic, also fantastic on the plasma.
4. Fast moving images are degraded on the LCD due to the 8ms response time of the pixels, but are super smooth and sharp on the plasma. If you are watching lots of fast motion and action shots, plasma is still the way to go.
5. The LCD has more processing options, including AMP, but these do change the character of the image in ways that are not intended, i.e. film looks like video, and anyway seem to be only partially successful attempts to catch up to the diplay quality of plasma. The plasma has a better picture out of the box and reperesents video and film as intended. Tweak away to your heart's content on both, Samsung has lots of customizability built into both boxes.
6. While minimized, there are still the inherent differences between LCD and plasma to contend with - field of view without color distortion of the LCD is restricted horizontally and really restricted vertically, while plasma is good side to side as well as vertically; LCD is way brighter (ideal for very sunny rooms,) plasma is way better with dark scenes. Surprisingly, the plasma did very well in a very sunny room for me, so long as it was not competing with direct sunlight on the set.
7. Both sets have moderate reflections, you would have to see which way they each deal with it if you might be sensitive to reflections.
I wound up keeping the FPT5084 plasma and returning the LNT5271 lcd.