I just recently took the plunge and purchased an N6 as well. Received it yesterday and can confirm I'm having the same issues w/ LTE. Moving from an N5 w/ lollipop, messenger sent text messages over LTE almost instantaneously. Now, I'm lucky if the text goes thru in 30 seconds and I had two text messages fail yesterday. Keep in mind, I'm lucky enough to have my towers broadcasting Sprint B25,41,26 and have been for several months. Definitely not a tower issue. I tried a PRL / profile update as well to no avail.
As an aside, this phone is huge. I'm really starting to enjoy it though the more I use it. The screen is beautiful.
Maybe 5.0.1 will fix it?
EDIT: Also, what the heck are you guys using for car mounts now? I used to use this mount for the N5 and loved it. I'm having a hard time even fitting it in my cup holders...