In the last year or so, new aero helmets such as the Giro Synthe ($250) and the Kask Protone ($300) have ushered in a new era in road helmet design. These helmets are more rounded-off at the tail, and more aero overall than previous state-of-the-art road helmets such as Specialized's Prevail or Giro's Aeon.
In addition, unlike Giro's older Air Attack, new aero helmets such as the Synthe are aero but -also- ventilate well. This is a paradox that is hard to pull off.
As for aerodynamics/quietness, if you compare the Synthe to the previous state-of-the-art, all-around, road helmets (Prevail, Aeon and before that, the Giro Ionos), helmets I rode in for years, there is no comparison. The Synthe is much quieter, especially when you put your head down during hammertime. The shaping at the tail of the old school helmets (Prevail/Aeon/Atmos/Ionos) is now obsolete.
The new Giro Synthe and Kask Protone helmets are "game changers."
Lastly, the ventilation of the Giro Synthe is different (and very effective). You actually feel the cool air running over your head -inside- the helmet (through the internal channels). This is brilliant. Does it vent *quite* as well as the Aeon or the Atmos? I dunno. We'll see this Summer. So, I might use an Aeon or Atmos on rides > 90°. But in all other conditions, I love the Synthe.
Versus the Kask Protone, I like the looks of the Synthe more (totally personal), and it's also less expensive.
In sum, the Giro Synthe is a great road helmet. Lucky BMC riders.