DH and I did Relate last year, and I thought they were fantastic. Up till then it felt like I was talking to a brick wall with Dh whenever i tried to talk about problems, and so I'd end up getting frustrated and shouting, then he'd just walk away, which infuriated me even more. As soon as we talked to our counsellor I felt she instantly could see the problems between us that I could see, but DH couldn't.
As the others have said, the first session is just to set out what to expect, try to identify what they can do for you, and if you want to go ahead. It's much better than just basic counselling (where in the past i've just felt they just listen and then just say 'well, what do you want to do?) because although the Relate counsellor is neutral, they will ask questions, advise on where you maybe going wrong, guide the conversation to keep it on track and they will if necessary give you little exercises to do at home, depending on what your problem is.
For example, because we had trouble 'hearing' what each other had to say, one of our tasks every week was what we called the 'tissue box thing'. Basically, we had to take turns to hold an object-could be anything, in our case the tissues were the nearest thing. Whoever is holding the object gets to talk to the other person for up to 5 minutes, telling them how they feel, etc and the other person HAS to listen without interrupting or contradicting etc. Then when they've finished talking, the listening partner relates back to them what they have said-to show that they have understood. Then you swap the tissue box over and do it again.
One of the rules with this task is that once it's finished, it's done, that's it. You must'nt mull over or argue about what's been said, or even talk about it, you just put the object down (tissue box) and carry on doing whatever you want to do. It's just a basic exercise to get you into the habit of talking regularly and really listening to each other.
I totally agree with butterbur in that I have no idea why DH did so well from having a third party there. He's always been the type to think counselling is a load of rubbish. But he responded to it and it really helped. We went for about 6 sessions, but you can do as many or as few as they think you need. Our counsellor told me that some people even go back for one session every few months, just to keep themselves 'on track' so to speak.
Good luck, I really hope it goes well for you.