I'm going to emphasise, yet again, that doing a subject via the DECV is NOT like having no teacher. You DO have a teacher, and can have as much or as little contact as you want. You'll have their email and phone number, and you can go in for tutes (it's in Thornbury, so it depends where you live). There are also seminars each term where you can meet your teacher. To answer your question, it's not really hard, but you have to make sure you keep up with the work and keep yourself motivated.
I know someone who did International Studies Units 3&4 via the DECV, and they enjoyed it. Some of the seminars were at the ABC in Southbank, and were apparently worthwhile. Also, you can complete 3&4 without 1&2, but I've no idea if any material overlaps within them.
My thoughts on choosing to do a subject via distance: If it's a subject you're interested in, then do it! If it wasn't for the DECV I wouldn't have been able to do Philosophy or Music Styles.