Canada's system is already so streamlined that I've never had a need to get a Canpass. Upon entry into Canada, leave your passengers on the boat, walk up to the dock and more than likely, you'll find a phone with instructions on how to call. (888-CAN-PASS) They keep an accurate database, so after the first time you call, they'll be able to use your MC numbers to lookup the boat AND passenger info. More and more often, agents will visit the docks to confirm the information you gave over the phone, but they've always been welcoming and polite. If you prefer to travel directly from US waters to a Canadian anchorage, then a Canpass is handy.
Our last marina was located a 1/2 mile from the border, so one year we decided to get I-68's. That's not so streamlined. They expire every year and require THREE passport-style photos (Kinko's only gives you two per shot). At least a few years ago, they didn't apply unless EVERYONE on the boat held one. That NEVER applied to us, so I never considered it again.
There's an OARS station on Mackinac Island, which to me seems like an easier way to re-enter from the North Channel.
After all this is said, I've never had a Nexus and don't even know anyone who has. Interesting concept though, so I hope you get some detailed replies.