Well I saw 2 liter ones at the LHBS and after bottling my first batch I am considering it too.. Looking at the ones they had, they were thicker than bottles.. so I would think conditioning in them would be fine. I didn't think to ask.
But it would seem easier to bottle in 9 - 2 liter growlers than 54 bottles, not to mention no capping, etc..
Its probably what I am going to do with my Octoberfest.