Like Tianna mentioned you'll run into that one guy that will ask you questions, the most important thing is to always be truthful. Nexus is based the honour system and they are very strict regarding being truthful, all it takes is one time and they will take the card away from you.
I'll give you an example of how strict they are, this happened to me a few months ago and I have never been stopped at the border before. If you are over your limit you have to fill out this form and just drop it off in a box, the box is in the Nexus lane so you don't even have to get out of the car. I was under the misconception that if you were over your limit or not you had to fill out this form while crossing the land border crossing.
Because this form has basic personal information to be filled out, I have that info pre-filled on my computer so I just print out the form before I leave. The time I got stopped I had left Montreal at about 6pm on a Friday night to go to NYC and on the way back I was at the Montreal border at around 2 o'clock. I forgot to drop the paper in the box so the agent said to just give it to her, then she started asking me detailed questions, which I found a bit odd for Nexus, She asked me when I left and came back & how much I spent. I answered truthfully. So then she said "You checked off 48 hours but you were gone less than 48 hours", I apologized and let her know that these were pre-filled and I hadn't noticed that 48 hours was checked off. Didn't matter they stopped me.
They searched my car, asked me for my receipts and told me to wait inside after they were done. They interviewed me for about 20 minutes, again I explained everything to them, they asked me how many times I've crossed before and if I had ever been gone for less than 24 hours before at the end they asked if I had any questions so I asked them if I would lose my card, and they said "that's what we're going to discuss right now" and asked me to wait in the waiting room.
15 minutes later they came out with a print out of the Nexus rules and asked me to read one of the paragraphs (stated about the severity of lying). Then they told me that they stopped me because I had checked off 48 hours when in fact I was gone for less time, It had nothing to do on how much I spent because I was within the limit of someone who had left for less than 48 hours.
Thankfully they said they weren't going to take my card away because they felt it was an honest mistake, they also said they put a note in my file stating that they explained the rules to me so if it happens again they will take the card right away. I also notice he had a printout in his hands of all the times I had crossed the border and when I had returned from the past year. They were asking me questions they already had the answers too to see If I was going to lie. I was detained for almost an hour and a half.
The Nexus card is worth getting especially for when you're flying, too bad all international airports don't have a Nexus line. When leaving or flying into Trudeau airport you can avoid long lines ,and the beautiful part when you return is you go to the kiosk, scan your eye and it spits out a ticket and you just go straight to your luggage area, don't even see a customs agent. Again like Tianna mentioned they do random checks but so far I haven't experienced that.
Applying for one is a bit of a pain and doesn't mean you will get accepted either, I know a few friends who haven't gotten accepted for the card.
