It's serviceable for some contrived use cases, but a long, long, long, loooong way from "fantastic".
I lived in Belltown first, and then Capitol Hill (right along the Broadway corridor) - both transit experiences were pretty shitty.
The transit is serviceable during off hours, but during rush it is an absolutely, completely unmitigated clusterfuck. All buses leading out of downtown are at crush capacity, and the ridiculous rule that back doors can't be opened means that at every stop there is a massive pile of human bodies as people towards the back shove and squeeze their way to the front to get out - only to run into crowds of people outside trying to get in.
If you work in the central business district, and live in one of the nicer "trunk line" neighborhoods, and you can stagger your work hours to avoid the worst of the inhumanity of Seattle transit during rush hour, you can get to work pretty well. If any one of the three conditions is not met, commuting via transit is going to be absolutely dreadful.
The only place I've ever experienced transit worse than Seattle was in the industrial slums of London, Ontario.