Hi all. I've just added the K845BT to my collection (which includes K550, K551, UE6000 and many others), and have a couple notes to share.
1. Under all conditions its sound signature is pretty K550/K551-like and not at all like the (to my ears) bassy and rolled-off presentation of the UE6000. (I don't have a UE9000 for comparison.) Wired and amped I think it's great for a sealed headphone. Wired and unamped, or unwired, I think it's very good, and I'm happy to use it on the go, but it's not up to my personal critical-listening standards.
2. It weighs noticeably more than a K551, but I like it better for mobile use, even apart from the Bluetooth functionality. On my head at least, the smaller earcups give me a better seal, as well as making for a somewhat less dorky fashion statement.
3. Potentially useful fact: the UE4000/6000 cable, with on-cord volume and play/pause controls and mic, fits perfectly into the K845BT's socket. So for the price of a UE4000 (currently very cheap, especially if you're not picky about color) you can swap out the K845BT's cable for one with on-cable controls and a right-angle connection at the far end.