My lightning 45 showed up today, and it's a pretty neat pack.
The suspension does a really, really good job of getting all the weight onto the hip belt, which is big and cushy. I also was initially annoyed by weird pressure of the lumbar pad, but the sensation went away completely when I loaded it up with climbing ropes and gear.
This 45L version weighed 38.7 oz on my scale (with tags), so not much lighter than the 60L. There is a 0.70 oz top strap and 0.35 oz ice axe bungee that can be removed.
It would be possible to remove the hip belt completely if I cut the end of the webbing that snugs the bottom of the bag to the hip belt. As many webbing ends are, it is sewn bag onto itself to keep it from slipping through the ladder lock. This double layer of webbing is too thick to go through the lightweight ladder locks on this pack. The load lifter straps are not sewn back, and as such they can be switched to the higher set of load lifter ladder locks. The bigger problem is that without the hip belt, there is no anchor point for the stay, and without the stay, the shoulder straps lose their main connection to the pack. So while the hip belt can be completely removed, the pack can't be used without it. Why do I care? I'd like it to function as a "bag with straps" for climbs with a long approach in the Sierras. My opinion is that it is unusable without the hip belt and stay. My fault for trying to get the bag to do something it was definitely not designed for!
With the hip belt as separated as I could get it without cutting the pack, it weighed 9.5 oz. The stay is 2.55 oz.
The side pockets seem as difficult to use as they looked with the compression straps tightened. These straps are very light webbing. I'm a little bit worried about the plastic rings that they thread through over long term use. I was able to secure a Z-lite (full length) to the side of the pack by putting only 3 of the 4 diagonal straps over the pad, and the other one under. I had to work to get the 3rd part of the strap over. In general, I don't think these straps would be good for heavier or sharp things, like snowshoes.
Overall, I think it's a really good pack, and a bold design. They have gone one step further than I've seen in terms of eliminating EVERYTHING that doesn't contribute to the load carrying ability of the pack. Obviously I haven't tested it, so this is somewhat meaningless, but I think it's only competition in terms of carrying capacity to pack weight ratio are the HMG packs (from what I have read) and probably external frame packs.
Since I want to avoid bringing a daypack for climbing trips, I won't be keeping the pack. The extreme design of the Lightning 45 is very cool, and I think that having toyed around with it for an evening has helped me understand what compromises I am and am not willing to make for my next pack. If I wasn't a climber, it might be a different story!
If anyone has any questions about the pack, I'd be happy to try to answer them.