I've been living with the Ergon BD2 back pack awhile now and I thought I'd chime in with my impressions so far. If you missed my first post on it, check it out here.
First of all, I still am amazed at how this pack feels empty when you ride with it, no matter what load I have in it. Weird! Well, weird in a good way. The only real pressure I feel is when the top straps are tightened and the buckle is fastened and the pressure under my arms is evident. It's nothing that I find as a nuisance, just different than you would expect. This becomes a bit less noticeable once you start riding, and to be honest, I'd rather have a bit of pressure there than on my back.
The biggest/heaviest load I toted in this back pack was the other day when I had the back pockets all stuffed and my three liter bladder filled and stuffed in it's pocket. Still no pressure on the back and it rode just awesome. Total freedom of movement while riding. This still is amazing to me and makes the BD2 well worth owning in it's own right.
Now not everything is perfect and of course, neither is the BD 2. The internal and external pockets are weird in that they are mostly all narrow and very deep. I stuck a multi tool in one of the two pockets inside the main pocket and it disappeared! I almost couldn't retrieve it due to the fact that my big ol' paw wouldn't go inside of the pocket very well to reach down far enough to grasp the tool. This could be a problem if you really need to get at something at the bottom of one of these pockets out in the field. I'd maybe split those long pockets into two smaller and less deep ones to make access to the contents easier.
There are two outside mesh walled pockets that are similarly shaped. Narrow and deep. I think something a bit wider at least would facilitate putting wetter things out there like gloves or head bands which would allow air to get to them. Maybe even a rolled up outer shell could be stuffed into this area if the pocket were a bit wider. As they are, they are perfect for longer items like shock pumps or mini pumps. The pocket that is part of the compression flap is perfect for your CO2 carts and a multi-tool. If it were a bit more voluminous, a tube could go in there, but as it is, you could maybe get a road tube in it with the other flat repair items. It would be nice to be able to get a mountain tube in there with that stuff, since this pocket is so easily accessible. That's important when conditions are poor, or if you are in a hurry.
Then there is the pockets I wish it had. The waist strap which bears most of the packs weight comes around you at a place that would make it super convenient to have some pockets for gel packets or energy bars. Maybe even large enough for a point and shoot camera. A pocket right there would be super, one on each side. Mesh, so you could kind of see what you're getting after. Another cool thing would be if you could get a satellite pocket that could be attached to the Velcro straps on the harness. The Velcro straps are already there to help hold down your hydration tube on either side of the harness, so why not? In fact, for the asking price of the BD2, it'd be nice if they included this. A cell phone, small energy/nutritional substances, or chain lube/multi tool could easily be carried here and obviously very easily accessed.
Of course, all of these wants and nits would perhaps make the BD2 even more spendy than it is, but after all: Ergon is going for the ultimate hydration/back pack on the market and not just another "me too" hydration pack. In that light I'd say there are some improvements to be made in terms of the storage and it's ease of use in the field. Once these issues are improved upon, I'd say Ergon will have themselves a winner in as far as the "ultimate cycling back pack" goes.
As the BD 2 stands currently, it is already head and shoulders, (pun intended) above the competition in terms of comfort. It has no rival in this regard which already places it high up on the food chain, if not at the top. I wouldn't consider anything else for a long ride unless it was the slightly less voluminous BD 1, which would eliminate dealing with the deep pockets of the BD 2 and be somewhat lighter to boot. However; for those that have to have a bit more gear on board, the BD 2 is highly recommended. Just be careful how you pack it up!