My daughter requested the backpack (D) and was VERY happy with the results. I customized things a bit and you can see photos on my blog: http://littlebrowntomato.com/2013/01/25/simplicity-1824-backpack/ .
This pattern seemed pretty straight-forward. There were a couple of times from Instruction point 19 and on, that I was confused.
For instance, at Instruction point 24, they want you to sew everything together, minus the zipper opening. Looking back, I'm not sure why I got confused, but I did. I also skipped making the pocket.
Would I make this backpack again? Probably, ... but just to conquer it without making mistakes. ;)