My 2c:
I migrated from the HR10 to the HR20 a while ago now. It's been nearly-rock-solid for me (one bug that was fixed a few months back). I'm a happy user.
I recently had the pleasure of getting a 2nd HDTV.
I hooked my HR10 up to it, so as to not miss out on any HD goodness. A few days later, I flipped DVR's, and put the HR20 on my new 62" DLP, where the HR10 had been operating for a week or so. Couple of points here:
1) The PQ was
CLEARLY inferior on the HR10. And no, I'm not talking about a MPEG4 to MPEG2 comparison. Same channel, same cabling, same program (I did it at halftime of the 2nd final 4 game). Bad pixelation and whatnot on the HR10. Much better on the HR20, and even better when I went to the MPEG4 feed. I've seen descriptions of the HR10 as having a "soft" picture, but never something this blatant.
2) The HR10 is slow. Even with 6.3c (the latest software), it's slow. Menus, guide, etc - ALL slow. It was a drag going back to it after months on the HR20 as a primary.
I also missed the mini-window on guide / now playing list. I know some hate it, but I think it's great. I watch enough live TV (sports, etc) to make it totally worth it.
The ONLY thing I find myself missing from the Tivo - suggestions, of all things. Why? Well, I have 2 kids, and they like their shows. I like my shows, my wife hers. They rarely coincide.
Suggestions always kept the programs I don't want every one of (kids especially) churning, and updating. Also turned me on to other things. It did this without adding a new series record. I'm now getting dangerously close to the 50 series limit on the HR20, and I'm hoping that gets at least doubled soon.
Anyhow - to answer the OP's question rhetorically, I'm going to retire my HR10 as soon as I can get an HR20 to replace it.