The Replay 4000 is far more advanced than any Tivo. I have two DirecTiVo units. The integration with DirecTV is the only reason I own Tivos instead of Replays.

Being able to share shows between units in the house is a killer app. There are so many times I'm watching Tivo in bed, but the program I want to view is on the Living Room Tivo. Sharing programs between houses is also awsome, but the speed of the transfer is the killer. Although... I suppose you could send stuff over night...

As for outfitting TiVo with ethernet... it is now easier since TurboNet is now out.