Just posting the Verge's review for now, but feel free to throw others in.
Gotta say, it is dumb as hell that the speaker can't be a speaker on its own. So if I'm listening to music, I can't pick up the tablet and move over to the couch without severing the audio connection. That is bananas. I'm sure it's a cost thing, given it'd need a chip of its own, but ffs.
Received mine today so that I can finally retire my old Surface 3. Not really impressed by the dock.. I mean, it works fine and sound is fine. Battery duration seems good and I hope it will stay like that. I don't see myself using it attached to the dock a lot.