But as you said: unpack a tar.bz2 file--how will i be able to do that?
Most likely your desktop enviroment(GNOME, KDE)'s file browser (nautilus, konqueror) has associated (.tar).bz2 files with an appropriate program.
In other words: double-click it
If you don't have any gui program for decompression, try file-roller. Or do
$ tar --help #to understand what happens :)
$ tar -jxf filename.tar.bz2
I fail to see the efficiency as an ISO shouldn't gain from being compressed.
In practice, you might be right (I don't know the data format or algorithms well enough to predict anything).
But in theory you're wrong--browse the compression faq:
Articles 9 and 10 (part 1) are quite funny