Just saw read this article last night in the latest issue of PC World (while I was waiting for my daughter to finish ballet). They're pretty hard on the Xbox 360 HD-DVD add-on. When I first bought the add-on, I still had the Toshiba HD-A1. I did some A-B comparisions, and the video quality of both were comparable (this was on my old TV -- a CRT-RPTV with both the Xbox 360 HD-DVD (component) and HD-A1 (HDMI to DVI adpater) outputting at 1080i). Anyway, it's an interesting read:
PC World Article