I bought this for a major road trip our family took. We spent 18 days on the road, drove 4300 miles, and hiked 2-5 mile on most of those days. Our baby spent anywhere between 2 and 6 hours in the carrier on about 12 of those days. She never complained when she was in it. She wouldn't even ask for a bottle. We had to remember to bottle her on schedule to make sure she didn't get dehydrated. Normally, she lets us know loudly when it's time.
She seemed to really enjoy being up high, and liked looking around at the scenery. Every time she would see me start setting it up, she would get excited. From her perspective, it was perfect. She was so comfortable in it that she would even nap up there.
For me, it was surprisingly comfortable. I could wear it for hours with no problem - I would forget I was wearing it. I wish the there was more storage on the belt - there is only the small zippered mesh pouch, but that's minor. You have to take it off or have someone else help to get stuff in or out of the backpack. The mesh back is great. Even at 90+ deg temps, it doesn't feel much hotter than not wearing a pack.
One drawback is that the high position of the child does raise your center of gravity. This makes you less stable than a carrier that keeps the baby lower on your back. I felt like I had to be very careful hiking on steep slopes. This is a trade-off though. I don't think my little one would have tolerated the carrier for hour after hour if all she had to look at was my sweaty back! I expected this and minimized hiking on steep slopes during the trip.
Overall, this carrier was key to us having an awesome vacation. The baby loved the trip, and was actually less trouble on the trip than her older brothers!