After a few weeks with the 3N1-20, I can just say it is one of the most convenient bag I have ever seen or used. There are a few caveats such as not enough side pockets or a water bottle pocket (there is the top compartment however), but I would still rate it a 4.9/5. When I am on the run and shooting track or in the town square, it is awesome to be able to just sling the bag forward, but when I was hiking a few months ago (used a 3N1-30), It was a 10-15 second process of changing the bag from sling to back pack form. The back side of the 3N1 is also very nicely designed, as the gaps between the plates offer a "circle" of cooling and it doesn't feel nearly as hot on your back pack.
One note- If you have more than 4 lenses (a 50-135mm, 2 small (or 100mm macro) primes, flash, and telephoto lens, I would suggest looking at the 3N1-30. Although it all fits well into the main compartment, there is absolutely no room for anything else such as chargers+ batteries which I have to put in the top compartment (which I usually put my lunch).
There is also a rebate going around ($25 off for a $95+ bag) Fortunately the 3n1-20 is around $99-$100. Here is the link.
http://www.adorama.com/pdfs/rebates/KA3N1_083109.pdf