I got my N5 this morning and activated new service with Straight Talk. Ported my number from Verizon, and I am getting LTE. I went to apn.straighttalkbyop.com and put in my phone and zip code (I had to select Nexus 4 if I remember correctly, since Nexus 5 wasn't an option). I entered what came up, powered the phone off and back on, and had LTE.
I've seen it switch between LTE and HSPA a couple times, but my speedtest on LTE pulled down 16-17 mbs and up was about 6.
I did have problems activating though. I entered everything on the website, and I got an error message saying the card I entered for my first month of service had already been used. Had to call customer service, and had a hard time understanding the gal I was talking to. I clled back to be sure there was indeed a month of service on my account, and the second rep confirmed there was. While I was on the phone with rep #2 my old phone quit working and my port was complete (took about 30 minutes to port from Verizon). Entered the apn info, and all has been well.
~Gary