BuddyTV: Is There Free Will on 'Supernatural'? 

Interesting article, although I have to disagree. IMO Sam and Dean would not have ended up where they are if the angels hadn't interfered. For example; in 'Faith' Dean was going to die and Sam took him to the faith healer- it was divine influence that made the healer choose Dean.
Another example; when Sam was going to kill Lilith, he checked the voicemail left by Dean and heard the false, very inflammatory one that was left by Zachariah instead.
In the Song Remains the Same, if Michael hadn't intervened and put everything back the way it was, the Winchester's choices would have changed everything.
Sure, their choices have led them down this path but who knows how many times the angels have nudged them back into line when they've strayed? And previously, they were ignorant of what was really going on. Now they know and knowledge is power.
The other reason that I think that free will is going to triumph- is Eric Kripke's commentary on Lucifer Rising goes on about how the most important thing is family and the angel's plans are crap. So I think Eric Kripke is definitely on Team Free Will. :)