The internet says that plastic bags are about as good as the pack liners, if not better because not everything goes into one bag; you can ensure water-tightness by individually wrapping every different item. Plastic bags can be used to keep specific sub groups of things together, like food or night time clothing etc.
If water-proofing is important, I recommend using a pack cover. Most packs come with one already in it, that you can take out and wrap around the entire tramping pack in case of bad weather. They are good when used WITH the plastic bags and pack liners, because while the large total cover protects from small amounts of water (such as rain), they will not protect things from falling in the river! I, personally, use both.
Pack liners are cheap too, but plastic bags are even cheaper. In fact, most shops would end up giving you the pack liner INSIDE a plastic bag. The liners are thicker than plastic bags, and not quite so easy to poke holes through. The best thing to do is use both. Pack liners also double as emergency survival bags; in bad scenarios, such as being stuck in the bush, you can sleep in them. They’re rather large, some being up to 3m long.