I used one in the past (though sale eventually fell through so didn't go through with the mortgage) - they were free and would have taken their fee from the commission from the mortgage provider somehow, but they still had access to the full market, not tied to anyone.
Then more recently I used another who charged upfront; I'm not sure they had any better deals than the other one, but were more closely involved already with the shared ownership side of things, so it was just easier to stick with them.
The biggest difference I think, compared to friends who found their own deals through banks, was that I didn't have to go through long interviews about what I could afford - my friend had to have an hour long meeting where they went through all their expenses, asked about them, played out various scenarios, asked how they'd deal with that, etc. I had nothing like that - just gave my bank statements to the broker, and that was it. Of course the different circumstances might account for it - I have no dependents, no partner, etc, so maybe it was easier to see my outgoings from my statement. But I'm also self-employed, which complicated things, so I was quite surprised when I didn't have to go through all that questioning! I assume it was because the broker did it all.
I'd have preferred to go to the one that didn't charge commission though, as £300 felt a bit steep to me.