I use the Sling XL as my office bag, and it has been amazing. It just right size for carrying my laptop, and everyday supplies around campus. No longer do I need to carry a bulky backpack.
Size:
The laptop sleeve is the perfect fit for my 15" MacBook.
The top front pocket is great for my phone, glasses, keys, and whatever small items you need quick access to. It will hold a 32oz Nalgene bottle, but you will not be able to fit much more than that.
The top front pocket and the main compartment share the same space, meaning whatever you put in the top pocket will be taken away from the main compartment. With a 32oz bottle in the top pocket, and a 15" laptop in the sleeve, you can still fit three 32oz Nalgene bottles in the bottom of the main compartment.
The rear compartment is the total length and width of the bag, but only about a 1-1/2 deep. Big enough for a conceal carry pistol and accessories, or in my case, a couple of the large Fade admin pouches.
Strap:
Is comfy; well padded, wide enough to distribute the load, but too wide (feels better than the 5.11 LVC8).
The three attachment points makes it very versatile. You sling it over either shoulder, or if your like me, carry it like a backpack on one shoulder. Unlike most slings you can make it comfortable to wear it how you want on either shoulder.
The large carry strap on the top of the bag, makes it easy grab and go.
Conceal Carry:
I cannot carry at work, so I have limited actual usage in the category. I can say the the front "consealed" pocket can hold a full size pistol with optics, however, depending on how full the main compartment is, it may not be practical. If the main compartment is fully packed it makes it difficult to reach in and grab a pistol (especially if the contents are rigid. Also, if the main compartment is full the firearm may start to print on the front of the bag.
The back compartment, on the other hand is perfect for conceal carry. Even fully pack, retrieving a firearm is easy. One of the zippers has "T" pull which makes it easier to open (I would suggest purchasing an additional "T" pull handle for the other zipper).
Overall, I am very happy with bag. The size makes it the perfect office bag for me. It's easy grab and sling on my shoulder. The padding is just right, so I don't even notice the weight. The discreet branding let's it blend in any scenario.