I have been in the US since 2000, about 9 TNs (and TDs.) (I apply early when travelling).
Rule 1: Have all your paperwork, and a copy for the border.
Rule 2: Be patient and polite. They have to do their job. You know you are legitimate, they do not.
Rule 3: TNs are temporary, up to one year. On each of my applications, it was a new application, and my employer was asking for up to one more year.
Peace Bridge Buffalo: Can be slow, they are busy. I was declined their once because I did not have my original degree, only a copy, (so I drove to the next border, no problem). I now take a degree that has been notarized as an original copy.
Niagra North: Big waiting room, I would hate to see it full, I did not have a problem.
Vancouver airport: Slow, made the plane by minutes.
Port Tawassen (spelling?) South of Vancouver: They do not do TNs, I tried.
Miami airport coming from Europe: Do not go there. It took about 4 hours. TN is not their speciality.
Lubec Maine: I think I was the first TN they ever saw. I showed them what to do, as they read their manuals. I do not think they would issue a first time TN, mine was a "renewal". They were really nice at his border, understandably slow, couple of hours.
Due South of Montreal: Not good. They had us wait, then some questions, then wait, then some questions, etc etc, then the "TN" guy was not there. Worst border of the bunch in my experience.
Toronto airport: Good, but give it time. I have not been in years, but they used to have a small office and did the paperwork quickly.
Next March I will be trying the Tijauna/San Diego border crossing.
Each of these borders can be different depending on the actual people working.