Less than foam core, and end grain balsa and prone to significant delamination and water absorbsion if the laminate isnt done perfectly. It is however the lightest core available per square foot. I've heard reports of nomex weeping water, and denting really, really easily with very minor bumps, but thats more a factor of the skin, and bond to skin.

So unless you've got serious skills, or complete and total confidence in your builder, dont use nomex. And its hard to get your hands on, the aircraft industry likes it, most of what you can get is the end of a run from some aircraft order.

I recommend foam core, corecell is good stuff - cheap, easly to get, easy to work with, wont absorb water, good dent resistance, bonds are easier, bends with heat, cuts easily. Only slightly heavier per square ft than nomex.

Bill