I own and fly with one; it’s a great bit of kit. Screen is clear in bright sunlight, it never overheats (unlike iPads), you can swap out the batteries (unlike iPads) and it is pretty bulletproof. Has some nice features like showing AGL.
Regarding the synthetic vision, connecting with the GDL 39 really is about getting traffic. The synthetic vision is really just a novelty for my personal flying, but it could be useful. I find the Garmin helicopter obstacles database to be pretty accurate and they show/warn on the 660 screen really well.
The addition of the GDL unit would give better attitude information, but otherwise works roughly the same.
Below is a link to the pilots guide and also to the new GDL 50 that has just come out
Garmin 660 Pilots Guide
Garmin GDL 50