Without an Xbox 3 this new 360 is just a services and cloud gaming machine right? No disk drive, maybe no HDD either (not sure if that was confirmed). So unless you have a good internet connection the device itself isn't worth having and we don't know when it is coming out or when those cloud features will be available, but as for an attachment to the Xbox 3 it is pretty cool because it allows for disk, and I hope digital, playback.
Funny it is almost exactly like the PS4's PS3 dongle that would allow playback. Maybe that still exists too, but if it is priced at $99 and not $149 it is a pretty good deal and keeps the cost of the new system down. I still think that people should just keep their current systems instead of bothering with these things, but maybe the saved space, one hdmi, and the cost of selling might justify it.