Well, that's interesting. I had a Tivo installed on Saturday and my first thought was that the SD picture was significantly softer (ie worse). In my case, this is displayed on a Pioneer PDP5080LX (so, 50" and full HD) via an Onkyo TXSR806 AV amp. I'd consider this fairly good gear, and it yielded a good SD picture from an SA V+.
To cap it off, the engineer doing the install then told me that the old V+ doesn't have upscaling.... It seems to me more like the old V+ had upscaling and the Tivo doesn't!
As for general feelings on Tivo, so far I'm not very impressed. When setting a series link it cuts off the TV picture and sound. When you're setting up a link whilst watching something else (men, multitasking? It happens!) this is annoying - even the missus said so.
Some menus are displayed in a different resolution which is untidy.
Also the lack of "i" functionality from the mini-guide is just downright annoying. Finally, and surprisingly, it's no faster (maybe even very slightly slower) than my old SA V+ when changing channel or indeed during general navigation. I think those daft confirm/deny noises will drive me half-mad, but at least you can turn those off
Hopefully these are all things on the "to be fixed" list. Even if they are part of the standard Tivo functionality, they would still be better put back as they were on V+ !
As a side note, I miss the display on the front (displaying either channel number or position within a programme you are playing back) but most devices seem to be removing this. I guess I'm just old-school!