If you do a search for Bioflex in the gear section of the site, there are around three articles about Bioflex. We're supposed to be getting an early production sample in for test some time before Christmas, so will have more detailed user impressions then.
For now, based in a brief waddle around in one at the Go Outdoors trade show, I'd say that the mobility of the back system is extraordinary, it really does move with you in an quite uncanny way.
On the down side, it's a little heavy and some people may be unsure as to the durability of the components - really though, it's hard to make a judgement call on that without long term use.
The Macpac is a very solid, effective, proven pack btw. In the end I guess it depends a little on whether you feel the added mobility of the Bioflex is worth the increase in weight. The top end version has welded and taped seams as well btw, so is waterproof.
Anyhow, have a search in 'Gear' - see tab at the top of the page - and you should find some more details.
