I have both a 501 and a Dishplayer. Now that we have a search feature on the 501, the only thing I miss from the Dishplayer is being able to use a keyboard for entering text for the searches.
I got the 501 free through the Club Dish promotion last fall. When I received it, I gave my old 4700 to a friend and connected my standalone TiVo to the Dishplayer. Since I'm rarely recording anything on the Dishplayer, it's been pretty trouble-free. The only problem I've experienced is finding the Dishplayer in its screensaver mode from a corrupted download or from the TiVo trying to record a suggestion at 3:00 a.m. Yes, the Dishplayer is a pretty good PVR as long as you don't try to record with it.