Having moved from HR10-250 to HR20-700, I say that even with the warts, the HR20 is better.
As mentioned, it's much faster. Searches, scheduling a recording, all sorts of things are faster.
[Purely personal opinion]Once you're used to the user interface, on the whole it's better. Fewer clicks to record; no confirmation screen for common activities; currently playing show in a small window while doing Guide, To Do List, and other activities - I like it. It's not perfect, and it's not better across the board (e.g., guide format limitations), but on the whole it's better. [/Purely personal opinion]
Some things (it appears), will never catch up. For example, the "Auto Correct" ( or whatever it's called) feature of a TiVo (that takes reaction time into account when you press play after fast forwarding a while and backs you up to what it thinks you were aiming for) is apparently patented and not likely to make it over to the D* DVR's.
Other things I do expect to see in the not to distant future (OTA for sure, hopefully dual buffers [which has an easy work around, so no big deal], etc.). Stability and consistency are also improving (although out of the box, my HR20 was better than my HR10, which was really getting flakey before I switched).
Ultimately, I think it will depend on your priorities. If you absolutely have to have a safe, solid, familiar box, the HR20 is probably not for you. Otherwise, I think it's a good move. I like to think that we early adopters have a disproportionately loud voice in how the box evolves in the early stages while the software is still maturing. Once the bugs are out, it will be much harder to get D* to change things to work the way you want them to.