Because junos is spreading rappedly over the world, because of the launch of the SRX devices. I had to make the switch also.
As mentioned before, i worked/work a lot with ScreenOS, and must say i don't mind to make the switch to JUNOS if needed.
JUNOS is a ritch and nice OS, the more you work with it the more you will love it. It has nice advanced features, which you would love to have in ScreenOS (but sadly never will ...)
So because of this switch i wanted to check if i understand the junos security solution and did the exames JNCIS-ES.
I passed and have a other Certificate I can hang on the wall (well put in one of the schelfs with the other ...)
If you want to go the JUNOS way. I suggest you look at the fasttrack.