Normally, I wouldn't post anything on this forum until it had been used enough to evaluate, and then only if it worked well.
However, this product is quite unusual for the reasons stated below, but has been replaced by a newer model, so won't be around much longer.
It is Bean's AT 55, 2012 model: http://www.llbean.com/llb/shop/78605?feat=AT 55 pack-SR0&page=men-s-at-55-pack
AFAIK, the last time a removable, flexible hourglass framed, suspended mesh backpanel pack was offered was by Alpine Designs with an ABS tube frame. Still have one, remodeled with Spectra 4 oz. white widow gridstop for all of the pack except the original backpanel.
The unusual thing about these packs is that the hourglass frame, aluminum tentpole-size tube in this case, is removable; and when installed, is completely enclosed by the pack. Unlike the Ospreys, Z55s, Deuters and others, bugs etc. cannot get into the space behind the mesh backpanel (Well, OK, noseeums maybe).
So I was surprised to see Bean's offering such a pack after so many years. Also, unlike the others mentioned, since the entire perimeter of the backpanel is sewn into the pack and supported by the frame, it can be much less rigid than the others mentioned, making it easier to bend forward and flex the back against the panel. (Bean's calls it a 'trampoline' backpanel). For this reason, I find this type of pack much more comfortable. It takes a lot of design savvy, because if the panel is too loose, the inner pack when weighted will rub against the mesh panel and your back, and if it is too tight, your back will not be able to flex, and much of the benefit of the suspended mesh panel will be lost.
Some people think the space between the backpanel and inner panel moves the center of gravity too far backward, making the pack less comfortable. However, when this type of pack is packed and adjusted, the inner pack comes quite close to the mesh, as close as you would want and still maintain the ability to bend forward without feeling the contents against your back. I think this impression derives from the earlier Ospreys, that had a quite large separation between the inner and outer backpanels, without much give when weighted. Also, larger zip pockets on a pack back often get used to hold more than just rain gear and fording sandals, and that will move the center of gravity back more than a little bit of air space behind the suspended mesh.
As you can see from the link, these are 55 liter packs, and weigh a few oz over 3 lbs., depending on the size and gender. The design is similar to that on packs often favored on this site, with the two side lycra bottle pockets, lycra shovel pocket in the center, single bag with extension sleeve cinched top, buckle attached top pocket with separate hidden wallet pocket underneath, and there is an additional zip pocket inside the pack against the inner back panel that will hold items to be kept separated. It also accommodates a bladder for those who use one. It also has a Kodra bomber top and bottom, rawhide 1/8" diamond mesh for the backpanel, lift straps, etc.
The only changes I plan to make to the pack are to sew on a couple large buckles to the ends of the hipbelt so it will tighten like an Osprey does, by pulling the two web ends forward at the hips; shorten the extension sleeve, sub separating buckles for the top lid attachment, add two horizontal straps across the front to hold my chair and items to dry out, and remove some of the excess webbing that is longer than needed. For a while, Bean's had a review posted that panned the pack because it had 'too many straps.' Scissor challenged, I guess.
The old models are selling for $155 and come in a large size for torsos up to 21" like mine, which makes all the difference, comfort-wise. The new 2013 models have 'less straps,' a zip instead of shovel pocket, and are a bit heavier, but have a 35 liter model. Price for the new models is $179.
I've been designing and building a similar pack for several years, but with a 'Jackpack' sidearm suspension and lower weight. Until it is finished (assuming it ever is) I'm hoping this AT55 will be ideal for multi-week treks, carrying around 25 pounds total, including pack, at start, and will let you know how it works out.
Please note: I have zero connection with Bean's - haven't even read the book.