I don't use a pack liner.
With my pack (a cuben fibre, roll top, top loader with no pockets) I prefer loading my pack with my kitchen in one dry bag, my bathroom/toilet in another and my wardrobe/bedroom in another. The dry bags have to be see through and cheap to replace.
I have ordered three types of lightweight dry bags ( or 'compression sacks')to trial. Two have ingenious valves on them so the zip-lock closure on them doesn't pop open when you compress the contents. The valves are simply modified seals in the base of the plastic bag (not added devices) so they create absolute minimum/zero additional weight.
Google 'Eagle Creek compression bags' for one (reinforced with nylon webbing)and 'global travel products compression storage bags' for the other (here in Australia). The third type is the aLocksac bag, these I have been using for the past six months. These are very tough and stretchy but don't have any valves on them - which I have to say hasn't been a problem.
All three types of bag profit from being stuffed with soft gear (clothing). The spongy nature of the clothing gives the bags a satisfying 'vacuum sealed' appearance and reassures you that they are air tight.
Using transparent lightweight dry bags does away with pack covers (cuben fibre doesn't absorb or hold water) and pack liners. I can swim with the pack towed behind me on the end of my whistle string when crossing rivers/canyoning. It also lets you compress everything separately, and find things easily, without having to unpack the whole back pack. I find this a boon when its raining.
The aLocksac bags have lasted three one week pretty tough expeditions so far. I've yet to try the others, I'll write up how I go with them later when they have had some use.