I had the Sony KDL46W4500 for a couple of weeks but changed to the Panasonic TH50PZ81B.
Absolutely no regrets. Before even powering it up the build quality is clearly superior. Once up and running it was love at first sight. The pictures appear to be more realistic with great shadow detailing.
I was concerned that at 50", normal SD broadcasts would be unwatchable, particularly as this still provides at least 50% of my viewing. However, after 3 weeks of viewing it's clear that it's the quality of the original SD broadcast that has the most impact. If a programme is originally recorded in HD, like "Little Dorrit", then even on SD broadcasts, this TV will revel in the detailing of the picture. If you try older broadcasts, (Early "Morse"), well then, artifacts become more obvious at 50", But this is probably the case with all larger TVs.
If you've got a room big enough to accomodate this size, I'd have absolutely no hestitation in recommending this TV.