Even though you've already had a ton of good comments, I'm going to jump in, too.
I've got the black signature Mandy and I absolutely love her. She was worth every cent and I don't regret the money at all. Here's a link to the thread I posted when I first got her. There are pictures of the bag on, and also a bunch showing what she can carry:
http://forum.purseblog.com/coach/my-new-mandy-103278.html
I've used it almost every day since I got it a couple of months ago. I like that I can use it in the rain and not worry about it. I also like that it's very slouchy so that it doesn't look quite as big on as it would if it were the same size but more structured.
I mostly use her for going to my office at school. On those days, I carry all my regular purse stuff (wristlet, mini-skinny, medium beauty case, huge
Coach sunglasses case, keys, cell, etc.), plus an agenda (LV small ring agenda), magazine, large bag with lunch, MP3 player, and small umbrella. Also sometimes a change of shoes. I usually walk from my son's preschool to my office, which is a half hour each way, and the Mandy has never been uncomfortable.
I also use her in place of a diaper bag, so instead of the office stuff listed above, I'd put in a bunch of snacks for my son, a few toys, a sweater or change of clothes, and a water bottle. Again, no problems with it being uncomfortable. The other day, I carried it with all of that stuff in it, on my shoulder, pushing the stroller, for about 8 hours straight.
I love her so much that I would buy another one in whiskey leather or maybe in khaki/white if I could afford it. I found the signature to be much lighter than the leather when I tried them out in the store but now that I know how comfortable the strap is, I think the leather one would be fine.
Hope this helps!