... I have to say I sold my 50xx because of the sluggish UI.
I've read about this a few times over at AVSForum, but I've never understood why this is true for some users and not for others. The UI has always been quick for me, both before and after a hard drive upgrade.

Changing channels took ~2-5sec and made channel flipping a tiresome chore.
I thought of it this way as well, at first. But these things really do change the way you watch TV, regardless of which brand you choose. I personally never watch live TV anymore. Or on the extremely rare occassion that I do, I "guide surf." But then, I was never much of a channel surfer before I had a ReplayTV.