Like anywhere, it depends on what direction you're going, and at what time of day. If your trip is opposite to the "commute flow," (e.g, heading *into* Seattle in the evening) it isn't that bad. Surface streets around Seattle are bad either way, but as the OP's destination is in the vicinity of the Center, they're going to have to deal with it one way or the other.
Bremerton has its advantages, but time isn't one of them. The ferry ride itself is ~50 minutes, plus up to another hour to wait until the next departure. The ferry terminal is ~10 miles from the airport.
As I said before, if one wants to avoid BFI, Renton is the nearest alternative. Auburn is ~20 miles away from downtown (as the crow flies) and it's freeways nearly all the way. There's a park and ride lot at the South end with busses that'll take you downtown. Paine Field in Everett would be a good alternative, and you can probably catch a bus to downtown Seattle. Harvey Field in Snohomish is a great airport, but it's 40 miles from downtown.
Ron Wanttaja