This stuff vs full HTPC?
First, thanks for the "raw data". Many reviews on these newer devices have been short of actual facts about what it's really like to use them.
Apart from the convenience of running through the setup using the same remote that came with the TV I'm having a hard time "picturing" what advantage this class of devices, either built-in or add-on, have over a full HTPC.
Now before people start going on about "exclusive offers" like HBO access let me observe that historically those sorts of features only lasted as long as a particular model's production run. A few years down the line (sometimes) necessary changes in the publisher's API or security model would render the app in question obsolete and unusable -- with nary an update in sight for what would be one of thousands of variations on the serving device. On that score it could be encouraging that two independent software appliance makers like Google and Mozilla are in the game now. Theoretically their interest in keeping these products updated past the hardware warrant expiration would be cause for hope. But neither of those companies has a track record in that area, although Google's performance when it came to phone O/S updates (where the Big G ceded the update process to carriers and their device manufacturer partners) does give one pause.
Embedded devices are wonderfully clever tech, but in the long run I'd rather invest in a known quantity: an HTPC I build myself and whose operations, including updates, I can personally manage to ensure its both secure and functionality.