I've been using a shell script by Olivier Hoachuck to download the WWDC videos and PDFs. The script comes in two variations, identical except that one uses curl and the other uses wget. It puts the downloads in a folder on your Desktop.
The nice thing is that it's a plain shell script. There are no Ruby or Python dependency issues, as I've heard there are with other similar scripts.
The script downloads the SD versions of the videos, which add up to about 52GB in size. To download the HD versions, globally replace "SD" in the script with "HD". I haven't finished downloading the HD videos, but they are about 6 times the size of their SD counterparts, so I'm assuming they will total roughly 312GB.
Compared to the videos, the total size of the PDFs is almost negligible at just under 400MB.
The SD quality seems fine for seeing everything you need to see. I'm downloading the HD versions just in case, and because my eyes need all the help they can get, and because hey, free videos.