This thing is tiny but the PERFECT size for my 30lb 15 month old.  We didn't put anything in it because we weren't trying to make her a pack horse, but it could fit a small bag of snacks, a set of keys, or other small things.  I love that it connects at the bottom of the backpack because then if my daughter gets twisted, its not around her neck.  The elastic portion of the strap is nice because it gives a little cushion if your child pulls too hard.  It is lightweight and breathable, and it was nice that it didn't cause her to have a sweaty back (which is the complaint I have seen from those fuzzy animal-shaped versions).
I gave it a 4/5 because I think the front cross strap sits a little high on the chest - I would prefer it sit under the nipple line, closer to the waist, for her comfort.  But other than that, it is a great find, and I'm so glad we got it!