It's too soon for me to give a complete and practical review because I only got my new ACPRO1900 about 10 days ago, but here are my initial comments...
I had Canon 200 EG (still have it) backpack that I really liked, but it wore out (prematurely) and I needed a replacement. I liked the size of te Canon and wanted to fins something similar in size. I went through the "Goldilocks" process of trying several options: some backpacks were too big and some were too small. Then I got the ACPRO1900 and it is just right.
It is now holding my Canon 40D DSLR (gripped) with EF 24-105L lens attached, my EF 17-40L, my Speedlight 580 EX II, EF Extension Tube 25II, Battery Charger, and some other little things all in the main compartment. I thought that the fit for my camera was a little tight at first, but it seem to be fitting pretty well.
There are pockets and compartments galore, and a good place to carry my iPad.
One thing I'm not so sure that I like yet is that within the main compartment there are actually two zippered compartments: an outer compartment for holding a laptop or other accessories and the main, sectioned compartment for the camera, lens, etc. The outer one closes with a separate zipper and also stays closed with a Velcro strip. The problem is that you have to unzip both compartments in order to get to the camera in the main compartment, so I have found it more convenient to keep the outer compartment closed with only the Velcro. The workaround, which is actually very good, is that the entire compartment area can also be secured using the quick-clips that are on each side, making it possible for easy access without having to use the zippers at all.
The main compartment has plenty (maybe even too many) dividers, and for some reason is completely removable, in case you want to use the backpack for some other purpose. It lets you relatively easily remove that section with camera and everything else so you can repurpose the backpack. Personally I don't see why they designed it that way - I bought it to hold my camera, not to go camping with or carry books. Plus, the extra padding as a result of the removable section adds to the tight fit of the camera when the grip is attached, which in my case is always.
The zippers were a little tight at first, but they loosened up very nicely and work great.
The backpack has a hood that easily flaps over the top, providing great environmental protection, and if that is not enough, there are integrated waterproof covers (2) that are cleverly tucked away in their compartments for easy access.
The outside flap opens easily to provide additional storage for accessories or personal items, and there are two zippered compartments on each side for additional storage with easy access. On one side, they have incorporated a way to attach a medium sized monopod or tripod, but I don't think it's practical to attach a full sized tripod. You can also attach a monopod or small tripod on the bottom using to loop closures.
The interior is Velcro, which Ape Case claims is to make it easier to see the contents. I guess that's true, but IMO it also makes it easier to get dirty.
One thing that I really like is that it has a good sturdy handgrip on the top and it stands up by itself. That, IMO, is a bonus. It also has very comfortable straps and the back is well cushioned and ventilated, so it is fairly comfortable to wear. It has a nice, wide waist strap, but I never used that on my old backpack, and I probably won't use it on this either. I wish that they had incorporated a way to tuck it away for when it is not being used.
All-in-all I like the bag so far, and I would definitely recommend it. After having tried several others, this one is just what I was looking for. I might change a few things, but it is well made and is the right size for my needs.