I usually don't sling and carry the bag -- I stuff my wallet in the little pocket on my Zolo and leave the bag in the car - but my lifestyle is very suburban, so I use my car just about anywhere I want to go. I know there are mamas who sling and have a diaper bag simultaneously, but I can't remember which bags worked best.
I didn't pay full price for my ISM - I bought it new off a diaper bag swap board. I am happy with this price -- about 35% off what is currently listed on their website. I bought a sample bag as a gift for a friend, too, and that was roughly 50% off -- again, I'm very happy with that price. I generally try not to pay full price for anything, though. Let's see, would it have been worth it if I had paid full price? probably. The fabric is very sturdy and the design is wonderfully simple. I was looking for something big, and my comparisons were a Lands End bag that wouldn't stay on my shoulder, the Mothership or Alpha 7 or the Loom Marsupial. ISM won out because it was cheaper than the Mothership or Loom, and I hadn't had much success with Lands End or Kecci (we have a Daddy Bag backpack which I absolutely hate because it is a huge black hole).
HTH! I can post pics of my bag stuffed with CDs and stuff if you'd like to see it.