I have the x-curve frame and solo bag and am extremely happy after two seasons of backcountry hunting in Alaska. I've packed up to 120 lbs (total pack weight) for 6 miles of muddy, steep downhill and the pack was very comfortable and stable. The x-curve fits my lower back nicely and matches the contours of my rather large glutes. You could say that the X-curve matches my curves.
I've added a top lid and access pack for some more capacity for backpacking trips. The Solo bag seems larger than it's claimed 3600 inches to me, but sinches nicely down to almost flat for light hiking days. The simple construction and functional straps were one of the selling points for me. I don't like a pack with all sorts of dangly bits and pockets.