A Nexus card DOES seem like a good idea, but out of curiosity, does it do anything for air travel?
The Nexus card allows you to bypass regular customs lines when crossing both by car and air from the US to Canada - and Canada to the US. When traveling by air, you skip the customs line manned by a human and go to a computer instead. That computer scans your irises to identify you and then asks you 2 (or 3?) basic questions. The computer then spits out a printed customs card and you're done (without waiting and without having to speak with a human).
They do randomly select a certain number of Nexus travelers for further inspection (1 out of every 20?). So the Nexus card by no means guarantees that you won't speak with a human.
In order to obtain a Nexus card, you fill out an online application and pay the fee (was $50 for five years when I applied). When your application is approved, you travel to the nearest airport with an interview centre and must pass an interview with both US and Canadian officials. Assuming they give you the green light - your card is then mailed to you a few weeks later.
Another relatively new feature is that if you are flying within Canada, they now have expedited security lines for Nexus holders (I used one last week when flying between Toronto and Vancouver).
Let me know if you have any more questions about Nexus.