I work at a major airport. This is my 3rd POE. I haven't worked the land border and can only talk about that in regards to what I've heard. I lean more towards the Customs (baggage and cargo) side of the house, though my experience with Admissibility Secondary (immigration) is "adequate." I've spent far too much time on the line, stamping passports, as you can't escape that part of the job, even when you get some seniority and get assigned elsewhere. I have been involved in the Global Entry, APC (kiosks), Mobile Passport, and most of the newer programs CBP has been rolling out.
I've posted a bit here on Flyertalk, but by the time I have something to say, someone else has already said it. There's some knowledgeable people here when it comes to immigration and traveling, and dealing with CBP. I think I've seen a few things that weren't entirely correct.
Anywho, let's get some questions going. I feel like there are things that can be explained from my side of the desk.
Link to last AMA on Reddit