I have used them for years when I am in uniform (Army Reserve). They not only resist water, but are extremely durable as well. During a year's deployment I carried the same notebook in the cargo pocket of my pants every day. It got thoroughly soaked a couple times, but I never lost a single page. I did have a nylon cover on it, which I think helped. I never have written on it in the rain, preferring to first get out of the rain, then do the writing!
As far as writing instrument goes, just about any pen will write on it well. I have found that medium point is better than fine point, and I usually use cheapo Papermate pens that work great. I have found spots on certain pages that a pen just won't write on without a lot of effort(maybe they got an extra dose of waterproofing?), but these are few and far between. I remember a comparison test in Popular Mechanics magazine a year or two ago that tested a regular pen, a pen touted for writing under water, and a Fisher space pen. The only thing the cheap ballpoint wouldn't do that the space pen did was write upside down.
To test that, I just took a piece of the RIR paper, put in a dish of water and wrote on it with my cheapo Papermate. Bottom line, they are great notebooks, and just about any pen will do.
That being said... I see no reason to use them for backpacking. A cheap Mead pocket notebook and a Ziplock bag weigh half as much and, like I said, I don't write in the rain much.