I really think these units are coming online too late to be significant (for long at least).
I bought the WDTV1 and loved the capabilities (still do)...until I upgraded my A/V gear to a new Samsung TV, and added a PS3. Now all of a sudden, I have two streaming sources instead of none...all in one fell swoop, making my WDTV irrelevant except as a secondary streaming unit for the kitchen or bedroom.
Had the WD units come out a few years ago, it would have had much more of an impact imo, and probably would have been the de facto standard for The Convergence (R). It's almost like they are too late...now so many devices offer streaming and/or transcoding built in...blu ray players, TV's, and of course game consoles.
I will be interested to see if these external hardware units will be around in a year or so. Now even receivers (granted, only the higher end ones for now, but I imagine it won't take long to get to the Best Buy level receivers) offer up WDTV capabilities.