I own a couple sizes of Sea to Summit pack covers that fit everything I own. However, more often than not, they are left behind.
In summer, I usually carry a poncho, so no pack cover required. In rare early spring or late fall trips, or locations that are reliably rainy, I may switch to a full rain jacket and pants in which case I will bring a pack cover.
I always double up with silnylon stuff sacks for non-essential stuff, and silnylon drybags for sleeping bag and clothes.
With a pack cover, my hip and shoulder belts get wet from rain of course, but they are mostly mesh/foam and dry fairly quickly.
I have soaked nylon pack bags themselves when not using a pack cover during prolonged rain. Not to the extent of problems with keeping contents dry inside silnylon bags, but I found it annoying none the less.
When rafting or kayaking, I use a full drybag pack.