I just bought my 2015 K900, and I love it. There are a couple of things that I'm NOT a huge fan of, but overall I think it's awesome.
1. There are only one set of Climate fans, so if someone turns it on high, everybody gets the same treatment.
2. Within the stereo, I have not yet found a way to keep a USB or the Jukebox on Shuffle after you turn the car off and then back on.
Neither of these are game changers, but nuances.