I took this route only from Bovec to Lago di Predil at June.
Very nice ride,you will love the open scenery of the Julian Alps while riding on wide curves. DON'T miss the Mangart Pass (see details in next section), however the road surface becomes poorer just after but it's not a big deal.
Echoing Stellios. A very good alternative to north/south on the 206. I really, really, really do not like cobblestones and the 206 has 20 miles of them.
The well known passo Predil that connects Italy with Slovenia. A great and quiet road with great views and some great corners (specially at Italy side). There is a new highway building over this area so probably you will meet roadworks (when I was there it's was near Mangrt pass).