I do not know anything about the interphone F2, but I do have and interphone F4 for about 4 years now. The F4 is a very good product. Blue Ant, one of the big names in blue tooth makes the interphone. The interphone F5 has been out almost a year now.
The f2s is a lone rider system, no helmet to helmet comms. The Bluetooth works great paired to my iPhone 4 no problem. Touch the button on the f2s and voice control the iPhone for calls and playing music. Volume is good on my end, I wear earplugs and can hear my music and phone at 70mph+. It's not loud but works. I'm told I sound clear with no wind noise so that's good. All in all this is a solid buy for someone who doesn't need helmet to helmet. I don't have Siri on my phone but I suppose I could try it with my wives phone if anyone is interested.