Colour me impressed.
Received the required h/w last week and finally got around to installing the s/w last night.
HD Homerun Dual = up and running in minutes
MythTV Backend = simple to install
XBMC Add-on = extremely simple to install
MySQL = simple to install
Creating the correct tables in MySQL = this took some fiddling. The MythTV setup would not run and didn't throw any errors. Finally got it working when I determined the TZ tables MySQL were missing and needed to be created from scratch.
Watching Live TV on the XBMC is great with my only complaint being it takes a few seconds between channel changes. Much much longer than my old PVR.
Recording simple shows appears to work fine but advanced scheduling may need some work. Haven't had time to investigate much.