I really like the Mark 55, it saves me a lot of trouble when designing landers and it gives a really good amount of control to them as well. I think that they are pretty viable, if not good, in some situations. I usually find the least useful part in the game that isn't structural to be the Place Anywhere 7 Linear RCS Port, because there are nearly no situations where they are useful, at least for me, being someone who never makes spaceplanes.