I have them, and I have used them. They are terrible for newborns, in my opinion, because the diaper doesn't fit very well, and emptying the insert into the toilet is terribly messy with breastfed baby poo. Later, however, when baby is a little older, it's not so bad. Of course, we also EC our DD, so the only thing we've had in a gDiaper is a little bit of pee here or there. And that's actually my complaint about gDiapers. The inserts are large and absorbent. When she has just a tiny tinkle, it only wets maybe 1/4th of the insert. To dispose of the whole thing seems incredibly wasteful (based on the cost of the inserts). Also - while I've never tried them overnight, I have heard they hold up great overnight if you place 2 inserts inside the diaper, even for super soakers - boys or girls. I've never had it leak. The design is actually pretty decent. Even better, if you like, rather than use the insert, you can take an infant or preemie size prefold diaper, fold it in thirds and use it as an insert ... turning the cover into simple cloth diaper cover. They wash/wear well, and the new design is much much nicer than the older design (and fit better, too).
That said, the insert (because it and the cover are wide) has a tendency to bunch between the legs, which could lead to some overflow - and which I would think would be slightly uncomfortable.
Like disposables, however, they have a stay-dry feel.
Hope that helps!