I purchased dark green spectra gridstop from Seattle Fabrics when they had that color. It sewed fine - no problems at all. No problem cutting it either, but I keep a scissor sharpener handy and use it frequently, so that the scissors are sharp enough to just slice through fabrics without much actual scissoring.
It weighs 4 oz per square yard, and became stiffer after extended use in the outdoors, while the fabric still at home on the role remained pliable.
After extended use in the outdoors, the pack also lost some of its waterproofness. That could have been due to the seamgrip sealer wearing out, but I doubt it. The PU inner coating is quite thin, and judging from appearances, I don't think it held up.
I am much more impressed by the 5 oz Xpac fabric (D-P VX07) available from Rockywoods, for it is much more waterproof and appears to be quite strong. It is 1 oz per sq. yd. heavier than the gridstop, so will add about an ounce to the pack over the spectra (the pack uses about one sq. yd. of outer fabric). But for me, the added ounce is well worth its weight.
When you consider a 2-3K cubic inch pack using about a sq. yd. of outer fabric, the difference in weight from 1.4 oz cuben or silnylon to 4.9 oz Xpac is only 3 1/2 ounces. For me, it is worth the extra 3 1/2 ounces, in an item that sees the most wear and tear of any I use, to have it much more durable and waterproof.
I looked at the around 3 oz cuben/nylon laminate from Zpacks, and don't like its stiffness. A pack with it would seem more like a pack basket to me.
Don't think you will be happy long term with .75 oz thin mylar cuben for a pack. It will be much more susceptible to abrasion, punctures and leakage than heavier materials, and will not hold stitches well. Suugest you read Joe Valesko's comments about this on his website, Zpacks. He is very honest about this, even though his products use this material, and he is talking about the heavier cuben with a thicker and therefore tougher and more water resistant mylar. If you are determined to use the light cuben, you should consider either bonding rather than sewing it, or bonding strips of nylon along the edges of the pattern pieces before sewing. I plan to do this with .75 oz cuben used in the end covers for a tent. Since the other tent materials are sil-coated, I will probably use GE silicone glue, but Hysol urethane glue would probably create a stronger bond where no sil-coated materials are involved.
Good luck, and please post when you've completed the pack, and again after you've used it a while. Thanks.