Very good. Much more stable network connection than my old Galaxy Nexus. I don't have LTE locally, but I did test it driving by an area that had it. 2 bars and 22mpbs down, 6mpbs up with 160ms pings.
I noticed that I now have HOME and ROAM networks available under network settings. Sometimes the phone sets a notification to sign into the carrier cell network. Doesn't affect anything and just opens a browser window if you click it.
Nexus 5 rooted with ST AT&T LTE SIM & full Google Voice integration