Absolutely love it, despite drawbacks

I bought this backpack because we are a family that is always moving. Either hiking, walking, shopping, or travelling, I loved the idea that I could keep all the baby's gear (and mine) together in a great-looking backpack and eliminate the need for additional gear (carrier or stroller).
Although I have a Deuter Kid Comfort III (which I LOVE), its frame-body is too bulky to toss in the car for daytime errands, and I worry about it being too big in crowded stores.

To fill the in-between niche, I found the KangaKid a fantastic alternative, and we use it daily. For travel, it is great because the baby bag and carrier eliminates the need for a stroller and other bags. It is big enough (if I pack very carefully) to even serve as my own backpack for travelling. The baby is very happy up where he can look around and never complains (even during the awkward on/off process - more on that below). I love to take him shopping this way - my hands and attention are free because the baby is safely tucked away and entertained up there, and he doesn't fuss like he does out of boredom in a stroller.
This backpack gave me back the ability to go shopping and visit with friends, even with a squirmy pre-toddler.

Comfort: While I definitely prefer my frame-pack for longer walks, the Kangakid is even comfortable enough for moderate hikes in a pinch. I have hiked with it for up to an hour, or been shopping with it for up to 3 hours (taking the baby out a few times to move) and I think that is probably the limit.
After a couple of hours carrying my 25lb baby, I am a little tired and have to be sure to stretch my lower back, but am otherwise fine. (I should add that I am a somewhat athletic 5'8" woman - I would not recommend this pack to someone who considers themselves out of shape or not very strong). I have no idea how it would fit a smaller person.

Support and padding for both the baby and the adult is quite good. The back is rigid, and while not as outright comfy as my frame pack, it has never been remotely uncomfortable. Same with the hip belt, which is not as sturdy as on a full-frame pack and so more of the weight ends up on your back (unlike a frame pack where you can put almost all the weight on your hips). This means that when I'm moving a lot or doing a strenuous hike, the straps are not as comfortable and my back takes more of the heat than with my big frame pack.

The only drawback I've noticed for the baby is that it lacks any place to rest his head when he falls asleep. If he's awake, he's usually happy and singing and waving to everyone he sees.

On/Off: As described in every review I've ever read about this pack, it can be awkward. I have learned to put the baby in it in a chair/backseat of the car/bench, and then pick the whole thing up gently by the top strap and swing it around. You do have to be fairly strong to accomplish this without help, but I've mastered doing it alone. Fortunately the pack does actually stand pretty well on its own, so in a pinch we can do this by sitting the pack on the ground.

My only complaint with the pack is that none of the pockets or zippered compartments are reachable when the pack is on. I would love to have just one pocket on the side where I could reach a cell phone/wallet.

I anticipate using this pack for a long time - as my baby becomes a toddler then a walker, I'll just use this as a backpack until he decides he wants to ride. When the carrier compartment is zipped up, you can't even tell it's there. Awesome design!