We are now on kid #3 with ours. If you are not accustomed to a back pack and 20-30 LBS, practice before you go for a few months. A good pair of walking shoes is essentail.
If you buy from REI, join the membership it will pay for itself. 10% rebate.
As for the backpack, the summit was the top end, with mulitple adjustments for me and the kid, included the sun shade canapoy and detachable back pack.
The pack and kids have made # of trips, I even were it instead of the stroller for hands free shopping at the mall or grocey store. The pack has been up on Mt Rainer, groomed paths, through the woods shed hunting (sun shade works well for keeping branches out of the kids face) to varouis festivals.