I use both the MyHD MDP 100 and ATI's HDTV Wonder, and I actually prefer HDTV Wonder in MCE 2005 to the MyHD card due to MCE's superior EPG and software. The MyHD card is nice however, and well suited to a less powerful rig, and also has superior connectivety (HDTV Wonder relies on the connectivety of your graphics card)
I've found the HDTV Wonder very stable, and recommend it highly for an MCE rig. Using it in ATI's software is more of a YMMV recomendation since you can't ignore the assorted issues, however I've used it in MMC with good resultas as well.
Using an HDTV card at 1280x1024, especially with an LCD is less than ideal however.