The most straightforward method is downloading via ftp from the command line. Just some notes about this: upon connection always issue the command
since some ftp clients use ascii transfer mode by default (which will break things when downloading non-ascii files). Then download the file MD5SUMS and check it against the iso you've just downloaded:
the line corresponding to the downloaded iso should be OK. Then burn the DVD using "burn ISO image" or something similar, depending on the software used. Following these basic and simple rules, I have never experienced any problem with openSUSE images. It is true that there are bugs in the installation disc (I experienced a mis-configured /boot/grub/menu.lst which I had to correct by hand) but I never heard about a truly broken iso image around the world. I hope this will help.