I have been studying the performance of three players: Samsung 2550, Panasonic 55 and Pioneer 51. In their day (just a brief two months ago), the Samsung and Panasonic were similarly priced and natural competition. The Samsung has the reon chip, which I had enjoyed in my Onkyo AVR, Onkyo HD DVD player and Samsung 5000. When the prices on the Pioneer dropped sub $300, I jumped on one out of the big following.
Here are my results based upon two Sony LCDs I have in my primary viewing area 52W4100 (120hz, 8 bit) and my secondary 46Z4100 (120hz, 10 bit) based upon outputting 1080p 24fps and bitstreaming codecs to my Onkyo 906s.
Blu Ray Performance:
Picture Quality: I would give the edge to the Pioneer, but ever so barely. The image seems a bit smoother and pops out on the screen. The adjustments are on a scale basis as opposed to the 55, which offers pre-set settings and manual adjustments. I noticed a slight green push of coloring in my 55 on my z series. This might be an illusion, but it did appear in movies that had a natually washed out looking image.
Load Times: Panasonic hands down. It loads BD java discs in nearly half the time as the Pioneer (45 seconds versus 1m30s to 2m on the pioneer)
Audio QualityI am bitstreaming, which puts the two on a level playing field (the Pioneer cannot currently decode DTS HD Master Audio). The edge goes to the Pioneer. Sounds are sharper, but this is only apparent on certain discs and is a slight advantage
SD DVD Performance:
This is where I think the difference between the two starts to show. The Panasonic is good, images are displayed well without jaggie lines or barcode contrast issues that appear to blink. That said, the Pioneer offers more detail. I do like the fact that the Panasonic can force 1080p24fps on dvds. The Pioneer cannot. I would grade the Pioneer an A with similar performance to the 2550 with the reon chip and the panasonic a B with performance that is good for most movies, but is not as sharp on my 52" tv. I could live with it on the 46, but the Pioneer is clearly better on the 52.
Panasonic is the hands down winner with universal HD audio decoding over analogues, profile 2.0, and firmware updates over internet.
Panasonic is the winner as well. It is more responsive, quicker loading and about a quarter of the size of the Pioneer. I had to rearrange an entire cabinet to accomodate the Pioneer.
In summary, both are top notch machines. I am torn between which one will ultimately become my primary source. If I had to recommend one over the other, I would recommend the Panasonic to the larger public. For folks with expensive displays and a bit more patience for quirks, I would steer them towards the 51 (only at current prices of course).