I have an MLD Ark and love it. I've carried packed weights up to the 25-30 pound range and it rides so nice.... no sore hips, no sore shoulders, no problems getting to anything inside it. I use a rectangle of blue walmart foam as a backpad and stiffener, but only because the foam is handy in other multi-use ways. The pack is comfortable enough without it. Water bottles can be a tight fit in the side pockets when the pack is stuffed full, but since I usually carry my sleeping bag at the bottom (inside a Riksak) and it's not super-compressed, there's always enough "give" down there to allow a water bottle to be fit into the pocket. Reaching the bottle is no problem for me.
I think what I like best (besides the quality, the design, and the comfort), is that the Ark is big. It's really easy to pack, with capacity for longer trips and its size allows me to minimize the number of stuff sacks I use. For smaller loads or as food is consumed it can be cinched down pretty effectively and still carry very comfortably.
I tried the MLD Exodus and it's a great pack too, but I found it was just a bit too small for my gear choices and packing style. For what I do, the Ark is perfect.