Are you referring to the medium Big Bag? If so, haven't used it yet. It's sitting in my closet, but the door's usually open so I can admire it every day. Been using my small BB (grained calfskin) since I went back to work recently. I carry all of my personal stuff in it, but would never think of sullying my lovely Celines with a crappy PC work laptop... besides it's not big enough. Work crap goes in a washable cotton shopper. The medium BB is more than big enough for work and personal stuff, but I imagine I would use it for travel... like a road trip, not a flight - need to control where I can put my beautiful bag and keep it clean! To answer your question, I think the medium BB is a great size and the slouchiness is definitely a plus: it allows the bag to expand and collapse. With its big 11" bottom, I believe it's comparable to the large Phantom - pretty damn deep. If the structure was really rigid, I would have found it too awkward to carry.
I love my medium BB. This is a bag for life. The smooth calfskin is delicious. I fantasize about a medium in natural calf. I haven't seen much out there that rivals Phoebe's aesthetic... except maybe Daniel's Bottega. I have no regrets, and I'm sure once I start using it, I'll become slightly less precious about keeping it pristine