I recently purchased a new one (with a $25 off coupon), and while I like it, it's not the end-all, be-all that I had hoped. I hate vests as well, I seem to end up carrying more flies and crap than I would like, just to be prepared. The sling pack lets me carry several boxes of flies and other stuff that I couldn't normally clip to my shirt or belt. You can easily slide the pack around to front and then slide it back, but if you do that you wind up getting chafed on your neck from the strap slinging it back and forth across your shoulder (moving to the front and then the back). I think Orvis really thought that people would sling it back and forth since they put the nipper attachment point on the bag, when it really should be on the strap that is in front of you most of the time. The new version is missing a feature that I think they should have included from the old version. It was a little detachable pouch that you could hook to the strap in front and carry your essentials (like a small fly box, gink and a small of tippet). You then only had to sling the pack to the front for some out of the ordinary object.
Since the new one doesn't have this little pouch, I end up putting my essentials in my front pants pockets (and attaching my forceps, nippers and Frog Fanny to the front strap of the pack - a lanyard around my neck also holds one small box of flies). I only sling the pack around and retrieve something from the main pack compartment when I have something I only need once in awhile (like 2x tippet or a streamer box, packable rain jacket or a sandwich).
I think if you plan it correctly and only have to reach in the pack every once in awhile and not every time you change a fly, its awesome. Its very light weight and is great if you already have a minimalist mentality but want to carry a few extra items with you for longer trips.
I have a friend who has the old style of the Orvis Sling Pack and for feature-sake, its better than the new version (although its not as rugged). If you can ever find one on ebay for less than $50, buy it. I recently watched a few old ones go for over $70 so don't hold your breath...Here is one that closes out on May 20: http://www.ebay.com/itm/Orvis-Safe-P...#ht_500wt_1413