I do not have a pack that has an airspace between my back and the packbag. My comments are based on reading and on my general experiences with packs affecting my center of gravity.
Virtually all backpacks move the walkers center of gravity rearward, since weight is being added only to the backside of the torso. The exception to this is something like a properly loaded Aarn pack with its additional compartments on the front. It is my experience that this rearward moving of the C.O.G. is a bad thing and that it should be minimized. This is supported in a lot of literature, such as Andrew Skurka's new book (pg 29).
I find moving the C.O.G. rearward to be a bad thing because it requires that the walker lean forward to counter it out. This forward leaning may not cause obvious problems, but I find that it contributes to overall back soreness, it pulls back harder on my shoulders (more soreness) and in more extreme cases I find it lowers my enjoyment while hiking because I'm actually leaning forward several inches and I wind up staring more at a my feet when I hike, then at the beautiful surroundings. In addition to soreness and lower hiking enjoyment, literature on the topic argues that it's a less efficient way of walking (see Aarn's website).
Besides an airspace, there are a lot of other factors that determine how much a pack affects your C.O.G (ie. pack weight and where the weight is). So while I haven't used a pack with an airspace, I have used many packs that adversely affect my C.O.G. to varying degrees. I'm not saying that an airspace is the worst thing, just that the less a pack affects your C.O.G. the better. I'd rather have a better C.O.G. and a more sweaty back, than a less sweaty back but accept the downsides mentioned.
While I'm on the topic, I want to mention that pack compression systems that compress the pack bag horizontally (ie. thinner) are preferred because they improve your C.O.G in addition to adjusting the pack volume. The ULA Ohm is a great example of this. A vertical compression system (ie. roll top) works well to reduce the volume of the pack bag but it doesn't improve C.O.G. which is a missed opportunity. Many packs have both (ie. roll top plus side compression straps) so a wise hiker may opt to use the horizontal ones before the vertical ones.