Don't be fooled by the looks of the REI Piggyback child carrier. What appears at first glance to be some kind of medieval torture device is in reality a very comfortable backpack that allows your offspring to achieve their full potential as a crosstraining partner.
As the LittleFlogger nears 40 pounds he becomes an ever-more-realistic training aid for carrying big mountain loads. The Piggyback makes it happen with adjustable everything for you and your passenger. Parents can switch torso sizes relatively easily. It's hydration compatible and includes lots of nice little touches like a tethered mirror, removable daypack and a good kickstand.
The high seat means a good view for kids. If you're under 5'2" or over 6'2" you should make sure it fits you, but you probably already know that. A separately available diaper back expands capacity. Quibbles: it's really big, there's a useful mesh pouch on one side but not the other, and no belt pouches for cameras, phones, etc. All in all a great value.