At the Guardian website, Steven Poole takes a look at a Leuchtturm notebook:
The most bruited feature of the Leuchtturm, meanwhile, one to which a circular sticker on the cellophane packaging is devoted, is that the paper is “ink proof”. They are not claiming that the mere addition of ink to the pages causes all other notebooks spontaneously to self-destruct; but that you can use a fountain pen and the ink won’t “bleed through” to the next page. In the spirit of scientific notebook-reviewing duty, I conducted a robust experiment. With a Rotring ArtPen Sketch EF (a bit like a fountain pen), loaded with sepia ink, I drew a grumpy man in both the Leuchtturm and the Moleskine. No bleed-through in either. Then, with a Rotring Tikky Graphic 0.3 (black ink), I drew a surprised sheep in each notebook. FAIL! The sheep’s solid-black face and legs were too much: a couple of spots of black ink bled through onto the next page of both notebooks. So I am, sadly, unable to report that the Leuchtturm is clearly superior in this regard.
Read more at Notes on notebooks | Books | guardian.co.uk.