It does leave every 30 minutes but remember, that's counting each side -- every hour from the mainland, every hour from the island. If you miss it, you'll wait an hour until it returns from whatever side it's going to and you depart. Not a huge deal if you miss it from the mainland, you can always go into a local eatery and get a bite to eat while you wait, but if you miss it from the island side there's nowhere in the immediate vicinity to go inside and wait.
I rarely ride on weekends and there are some places I would never ride weekends, but this is one place I would. Traffic is generally not very heavy, as there aren't that many full-time islanders and the ferry can only bring over so many riders at a time! Even when there is traffic the riders seem to be more of the trail cruiser types and generally pretty well-behaved.
Depending on what the temps are, it can be a cold ride over on the ferry (if you go via sled and not your car/truck). But, there is an enclosed and heated area you can go into during the ride if you choose.
