DH and I went to (still do!) Relate. There was a bit of a wait before they could get us an appointment, but once we were 'in', we had a weekly appointment. In reality, it has worked out to about every other week - mostly down to the counselor's days off, bank holidays, etc....Getting DH to go wasn't THAT difficult, but I did have to insist on it, as he didn't see the point particularly.
DH and I have both (in the past) had separate counselling - DH saw a grief counselor after First Wife died, and I saw a counslor when I was having some health and personal issues. The stepkids have not had any kind of counselling, and refused to go to any after their mother died.
Our Relate counselor did want to meet them, and thought it would be good to see them too, but in the end, it has been just DH and I. The kids went along once and the Relate counselor met with them separately, then we all talked together at the end. That was a good session, and quite helpful I thought, but the kids have not wanted to go back. I am OK with that as really I would like to keep the focus on DH and I's relationship - after all, usually I am not so much in conflict with the kids as in conflict with him over how he deals with the kids, IYSWIM.
The Relate counseling has been a mixed success. We are actually trying right now to decide if we will carry on with it, or if we have gotten all we could from it.
Honestly, for me it has been most helpful in that it provides a reality check! Before we were seeing anyone, DH would just tell me that I was over-sensitive, or he'd deny that anything strange was going on. It took an outsider to make him see that he was being a DisneyDad, or to realize that our couple relationship needed more time and effort, or that his relationship with DSD was unhealthy. There were things which seemed obvious to me, but which he couldn't hear from me without becoming defensive, but which he could hear at Relate. I don't think he would have ever made the shift to wanting to make changes, without the counselling.
As for me - the Relate counselor mostly just reminds me of how far things have come, and tries to get me to focus on that instead of the things that are still problems. I'd rather we spent more time on practical strategies, instead of just 'acknowledging' our progress - so I get frustrated - but I do think that the counselling was helpful, and without it, we probably would have split up.