While you don't need to be a frequent traveler, you do need to have a motivation to go to the airport at least once while not traveling (to be interviewed and have your iris scan) and to spend $50. I think it justified itself the first time I used it. You skip the entire lineup for customs and immigration and instead spend a few moments at a kiosk. It's marvelous. Occasionally you are sent for random inspection, and if you're breaking any rules (and the Nexus rules are stricter than the regular border rules) you lose your card. Forever. Trust me, that motivates you to stick to the stricter rules.
You have to be both a citizen and a resident of the US or Canada. I was told to bring things like Hydro bills to prove I lived here. As well you have to tell someone why you tend to cross the border. There's a background check and some general investigating of you in advance, plus the interview. All well worth it.
In the years since Nexus was introduced the benefits have increased. For example, YYZ (and perhaps other airports) has a Nexus security line which saves you 20-30 minutes of waiting and guarantees you won't have to go through the nude-o-scope (I think they are also less demanding about shoes and belts.) I was also automatically enrolled in TSA Pre because I have Nexus. The time savings really add up.