Eagle Creek PackIts are excellent those are what I use and have used for a while. I got some old ones that are slim and longer and have a mesh front that are really fantastic for clothes I want to air out or for smaller bags but I also have what is now the PackIt Isolate (which I forgot what they called them back when I got mine) They also made a dual sided one that is perfect for clean and dirty clothes.

I have probably had the Eagle Creek stuff for 14 years now and have used them a lot and they have been great and still work well after that time despite being ultralight, they are super well built and I have overloaded them before more than once.

Yes the disposable plastic bags could work and I might use them for small items (like soap and such) but durability and longevity and ease of use is key for me. I have had enough disposable bags fail at the slider or the "zip lock" or whatever it is called and that doesn't work for me for my heavily used stuff. Considering my Eagle Creek stuff is mostly 13 years old and the stuff I just got is probably older knowing who it came from (it did need some cleaning but it looks almost new) it was a fantastic purchase initially and I know they are well worth it.