I returned the Kata 3n1-10 bag because it was just a little too small. I also realized that having the option to carry a laptop with me (without carrying a separate laptop bag) was valuable. So this led me to the Lowepro Fastpack 250, shown here behind a Nikon D90 with 17-55mm f/2.8 lens. Hopefully this brief review with pictures will give you a sense of the bag's size and layout in comparison to some common camera equipment.
This bag is a bit larger (mostly in width) and noticeably heaver than the Kata 3n1, given that it holds a lot more. The back has mesh padding which helps if you're sweating - this was a one nice feature that wasn't included on the Kata. Straps are thick and well padded.
Unlike the Kata, the Fastpack only has a quick access flap on the left hand side of the bag (from the perspective of someone wearing it). This makes sense if you're right-handed: slide off the right hand strap, pull the bag around with your left arm and pull the camera out with your right hand. Those wanting a flap on the right may prefer the Kata, since it gives you an option.
There's a flap with two quick release clips over the main camera/lens compartment, and a mesh pocket for water bottle on the right side. Underneath the flap, there's a slim zippered pocket which could hold filters or something else that's fairly flat.
Inside the main compartment, you can see the D90 with 17-55mm attached (no hood) and immediately to the right of it, the Nikon 12-24mm f/4. If I took the 12-24mm out, I could fit the 17-55mm with its hood attached. The D90 doesn't fill up the space for the camera body - I wish it were a little more snug but they've allowed room for larger FX bodies as well as DX bodies with a grip attached. Above that there's a narrow compartment that could hold a couple flashes, but for now I just have the battery charger and AC cord on the left and the lens hood for the 12-24mm on the right. Below, I've setup a divider between my 35mm f/1.8 on the left and the 18-105mm VR on the right (with lens hood attached). The dividers are sturdy and the compartment is well padded on the outside - equipment would be well protected here.
This photo shows my Dell Latitude D630 sliding in to the laptop pocket. This laptop only has a 14" LCD so there's a fair amount of extra room (the bag is rated for a 15" LCD). Even without a computer, you could throw some books or magazines in there.
The top section has a small zippered pouch on the outside. Not a whole lot of room in there.
The flap on the top section is not very well padded compared to the bottom section; I wouldn't put any optics or sensitive electronics in here. But there is plenty of room for other accessories, maps, lunch, etc. I've shown it with the 17-55mm for a sense of scale.
Overall, I like this bag. I could see that I was going to outgrow the Kata 3n1-10 very quickly, so this is a better solution for storing and carrying my gear. The 3n1-20 is probably closer in size to the Fastpack 250 (Kata also makes a model with a laptop compartment the 3n1-22) but I don't see the utility in having a sling bag option for a bag of that size (that much weight on one shoulder is not going to be comfortable for any length of time). Also, I felt that the clips and zippers on the Kata were not as easy to use as I expected, so I opted for the Fastpack instead. The only downside is that once you get it loaded up with gear (especially a laptop), it gets really heavy - not the sort of thing I would want to take an all day walking tour with. However, it'll get all your gear on the plane, and you can leave what you don't need for that day in the hotel.