Mares BCDs have a decidedly mixed reputation when it comes to integrated weights. Complaints that a weight pocket just falls out of place, especially in the prone, face-down position, are a fixture of virtually every consumer review thread on a Mares BCD I've ever seen.
That said, lots of people report no problems with their integrated weights at all. It's one of those things where I think if you are considering buying a Mares BCD, go do your homework and see how comfortable you are afterward. There is enough static that it ought to be taken seriously, but not so much that it is a closed case.