I've only done 2 batches, but I got around 80% efficiency. I mill pretty fine, gap is set at .030" on my barley crusher. As Jake Keeler says, I goose it to keep the mash temp in line. I mash in my kettle, have a steamer rack on the bottom (so I don't scorch the bag or grain) with a chain going up to the handles so I can pull it out after the mash. I like it a lot. Today will be my first time doing full volume mash. The last 2 batches I did partial volume mashes, like I used to with a cooler setup.
Brew in a bag is a kick ass method. Oh, and I do 3 gallon batches, so it suits me pretty well.