There are different weights of silnylon. Most commonly referenced here is 1.1 oz (uncoated weight) ripstop siliconized nylon, which will weigh 1.3 to 1.5 oz per square yard with silicon impregnation. Exact weights vary due to manufacturing tolerances. Slightly heavier versions are also in use commercially, including floors of some lightweight shelters and in some packs.
The 1.1 oz will abraid relatively easily compared to much heavier traditional nylons like oxfords. Silnylon is casually considered to be waterproof, although most versions don't meet the military definition of 25 psi. It will reportedly 'mist' lightly in heavy rains and can soak through if under pressure (i.e. kneeling on it, under pack straps, or similar).
Silnylon makes pretty good tarps, ponchos, and low-abrasion portions of packs. It is not breathable, so is has condensation issues when used as a tent or other enclosed shelter without ample ventilation. Those doing heavy bushwacking or mountaineering will go with more durable fabrics for packs due to silnylon's tendency to puncture, rip, or abraid, but it is in regular use for on-trail backpacks.