Summary: a lot of storage space so you needn't take another bag, the litte one is strapped in very well which means even if they wriggle you don't need to worry about them, all the straps are well padded and the chest strap gives extra support. Definitely worth paying the extra, especially for longer walks.
Pros: fully adjustable, weight is evenly spread across shoulders and hips, comfortable for carrier and little one, large amount of storage, sun/rain canopy useful
Cons: although we can adjust it fairly easily it's a bit too heavy for me (I'm rather small and weak!) but my husband finds it fantastic and as we've just made the transition from front to back carrier it's so much more comfortable