You can get the slingbox functionality of the ViP922 by simply using a slingbox with a ViP722K or ViP722 or really any ViP.
The TV2 output on the ViP922 does not work right now, might not ever work. Might just be that the Sling function is considered a replacement. Could be that even Dish doesn't know. Or maybe there's a "ViP922K" in the future, adding this feature. Of course, TV2 output might be of no interest to you.
It is doubtful, but possible, that Dish will ever put the ViP922 GUI on the ViP722K.
And we might see an internet browser and other features come to the ViP922 that the ViP722K will never get. The other ViP series boxes almost certainly will never see these.
The new ViP922 GUI is it's own creation, independent of Sling.
Is the ViP922 the most cost effective technology? No, and probably never will be. I suspect they want this to be their Cadillac while they also sell Buicks and Fords. And one day, they may add a Lexus.
FWIW, if I were coming to Dish today, and was willing to put up with the teething pains of a new product, I'd go with the ViP922. But that would be only if I thought I'd use the remote viewing and it worked well.
Maybe in 2-3 months many of the problems, such as EHD support, will be worked out.