For years I had several Exped bags of varying size. No problem with them.
I then tried an Alpkit which was equally as good and a lot thinner/lighter.
Then, much against my better judgement (snobbery) I tried a cheap one from Sports Direct - which was made by the 'new' Karrimor. As a result I then bought several of these Karrimor sacs as they seemed very tough/durable and haven't had any waterproof problems with them either.
Just shows how you can be wrong about things !
As vghikers comments - I used to rely on a single large pack liner, but ditched that in favour of smaller ones for specific items. Each is of a different colour so that I know what dry bag to look for when rummaging in the pack.
I now use the old pack liner to stuff all my dirty clothes in