This is the "cult" kid pack

Tough Traveler is the underground hit for kid carriers. Made by a small company in upstate NY, these are MADE IN THE USA and are quality. They are more expensive than their counterparts (Kelty, Evenflo, etc) but they are much more durable and are just a more solid pack.
The great features include adjustable shoulder straps (so you can wear the pack differently than your differently-framed spouse), and my favorite feature is a rain hood (sold separately) that stretches neatly over the top of the carrier and has a clear flap in front to allow the child to see out even in the midst of rain. The pockets are very spacious and there's a netting piece on the back for quick stowage of important stuff like kid bottles, pacifiers, etc. I still carry my daughter in it and she's a good 30+ pounds and it's not close to maxxing. I think you can go up to 45 lbs(although at that point, make the kid walk). We've used ours for concerts, fairs, hikes, etc. for our first child and are warming it up for #2. My daughter often prefers the Tough Traveler to a stroller (much to my back's dismay). The final nice feature is that it folds down to about 4 inches thick very quickly and easily, so it stows very simply.
Whenever I see someone with a Tough Traveler, I always ask how they like it and their eyes light up and they proclaim its virtues. When I ask people with other packs how they like them, they typically say, "Well, its ok" ho hum. I highly recommend this kid carrier.