I just got the 51D550 just in time before Christmas and had been toying around with it. Coming from a large screen projector setup albeit an obsolete 720p model, I am quite pleased with this Samsung full HD plasma which is a godsend when it comes to playing full HD material. Now I realize why folks keep saying there is no going back to standard definition once one has experienced full HD. Now all my DVDs look awful in comparison to full HD material, although I have to admit I'm still using a cheap DVD junk player that does not upconvert, running in 480i resolution via component. I have some HD movies on HDD and hooking it straight into the Samsung via USB inputs at the side of the TV these movies all look great.
Now I'm wondering if I should salvage my large collection of DVDs by getting a good blu-ray or DVD player with good upconverting chips. At any rate I do realize that optimized DVD playback will not match HD material.
I have yet to try the 3D feature of the 51D550 but initial impressions with the 3D glasses were not very encouraging. Right now I am just enjoying 2D as it is. The resolution of 1080p is a welcome note for me having got used to 720p all these years, although the 51" screen is quite a bit smaller than the 80" projector setup in my Home Theater system.
I agree with some comments on the reflective panel of the 51D550 and it is best to watch it in a room with dim lighting. In a bright room it can be a problem. The use of thick curtains or blinds may help.
If this Samsung 51D550 is trouble-free for the next 3 years, I'll be getting a 65" model(or larger) from them in near future. 51" is not exactly that big these days. Just a bit worried manufacturers may not produce plasma several years down the road(I still prefer plasma over LCD/LED mainly on picture quality, apart from the lower price tag).