I researched and shopped for a long time. I had settled on the LN52A650 and dummy me purchased the LN52A550 by accident.
I shuffled some paperwork and blah blah blah, I almost exchanged it but decided that the 550 is fine for me. Had no real interest in the 120Hz and it has excellent response time and a very respectable aspect ratio. I love this tv. I have my PS3 and my Mini hooked up to it and everything looks so good. The black levels are fantastic and the price on them are reasonable. What it comes down to is buy what will serve your purpose best and what you can afford. A piece of advice is buy the biggest you can, nobody ever regrets buying too big a tv. I have mine in my room and I think I want to go bigger now.