I don't have personal experience but from various reviews online (linked below), the paper does not perform very well with fountain pens. Several reviewers noted that ink feathers and there is a bit of bleed-through. Additionally, many of the reviews note that it's difficult/impractical to write on the back of the paper––which ultimately reduces the viable writing material by 50%. Love the idea, but it seems like the product has some issues.
Reviews:
http://www.dragoncompany.org/review-bull-stash-notebook-travel-stash/
http://www.fountainpennetwork.com/forum/topic/287927-bull-stash/