I ride the ferry every time I visit: it's the cheapest scenic tour available. Views of the Seattle skyline are fantastic at any time of day, in any season, in any weather. Because of the vehicles, the vessels are very heavy and the ride is smooth, smooth, smooth. There are both open and enclosed areas (wear a warm windproof jacket if you want to stand on deck and take photos), food service and restrooms. For sightseeing, I prefer the Bremerton route, and I daydream of living on the water.
This past visit, I actually used the ferry service. The system is efficient and there is no way that you can become confused or make an embarrasing mistake that would delay things, either boarding or disembarking. Plus you still get to sightsee!
The Washington State Department of Transportation publishes anything you could possible need to know about the ferries online, and their phone app is great as well.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.