I got the 51swx20b about two weeks ago. So far I'm pretty happy with it, although I can't say I've really had a chance to evaluate it fairly. I'm in an apartment that is too small for this TV, and am moving to a house in a couple of weeks. After that, I'll be able to sit at a more proper distance from the screen, as opposed to the ~7' I'm at now.
Anyway, the TV is nice. DVDs are excellent. Cable TV is pretty bad, but you get used to it. I had a Sony 36HS500 direct-view before this, and cable was awesome there, so it's a big change.
Video games are also somewhat disappointing on this TV. By this, I mean that the screen is large and high-quality enough to show the imperfections of the games. The aliased edges in XBOX games are especially noticable. Dead Or Alive 3 looks incredible, though, so it sort of depends on the game. The big size of the screen makes up for the aliased edges, though... biiiigg is gooood.
AFAIK, there are very few TVs that display a 720p signal natively, and they are super high-end. The Hitachi accepts a 720p signal and converts it to 540p or 1080i, so you will be able to view a 720p signal no problem (I think ABC is the only channel broadcasting in 720p anyway). Actually, this is what the SWX does with all signals: converts them to 540p or 1080i (you set which it converts to). This is in contrast to the 51" Sony that I also looked at. The Sony downconverts 720p to 480p. This was a big part of the reason I didn't get the Sony. That, and the extreme glare from the screen guard. From what I could tell, you can't remove the screen protector on the Sony. You can on the Hitachi.
Another nice thing about the HItachi is that the screen protector is low-glare. They also did some thing to make it so you can view the Hitachi screen from a greater angle on the sides. It seemed to me that this TV was as close as I could get to a direct-view tube in the RPTV world.
I'll wrap up now and say that I think now is a good time to buy this TV if you're looking at it. They seem to be closing them out. I went in to Circuit City to look at one, at which time it was 2300. I left, came back in about an hour, and the price was 2000! Plus, they were giving a $200 gift card and free delivery. I think in the CC ad this week, it's $1950 with no gift card.
Feel free to ask any questions.