Because I use mine as a carry on, I've never had it out in the rain for more than a few minutes while running from home to car, and car to parking shuttle, and it's never soaked through.

I was curious, so I dug it out of the closet just now, laid it on its back, and poured a few ounces of water onto the front. It beaded up initially, but I left a small puddle on the front and it soaked through the exterior and the lining into the front compartment in about a minute. I must say, that disappointed me, but it's always been fine for a few minutes of transition from house to car to parking shuttle in light to moderate rain. During those transits, I never roll it on a rainy parking lot, I carry it by the top handle or by the shoulder strap I added, to keep the wheels away from any puddles.