After spending over a month in my pocket, I would say this Field Notes graph notebook has held up pretty well.
To test the paper, I used a wide variety of instruments and inks to test show-through, bleed-through, feathering, and overall quality. As you can see, there was some feathering with the inkier pens.
Even with quite a bit of showthrough, the ink did not leave anything on the next page.
The saddle-stitched binding has held up pretty well considering it has spent so much time being sat on. I've folded back the cover quite a few times, too. Overall, this notebook held up quite well - better than Moleskine, which has turned into pulp with the same abuse.
The Field Notes notebooks are not a bad value. If you want something that is more fountain pen friendly, I'd go with a Clairefontaine/Rhodia pocket notebook but, given the price (3 for about $9.99 + shipping), they are a decent notebook. Because of the shipping costs, I'd consider signing up for their newsletter, and they will let you know when they have special offers. When I bought my 3-pack, which included a blank, a line, and a regular notebook, they added a free 3-pack of the graph notebooks, my personal favorite.
They have a pretty cool website, too, so go check it out. Field Notes