We are doing raised beds and have read/heard conflicting stories of how to use this. And can we use straight mushroom compost without top soil?
My Advice
I wouldn't do that. Mushroom compost is great for adding organic matter to your garden beds but soil is the long term ingredient you want.
You can use say 30% mushroom compost. The balance could be all soil or if you need to reduce the weight of the material add 30% vermiculite or perlite. Vermiculite is best for helping hold water in the soil, Perlite is best for improving drainage.
So a good mix would be a third compost, a third soil and a third either vermiculite or perlite.