I've been using the Scala G4 for a few years now.
Battery life is very good, I can turn mine on in the morning and leave it on all day without a problem.
They have an impressive range. I actually have no idea how far they will work, but I can still communicate with my riding buddy even when we are out of sight of each other (e.g. when one of us makes an overtake and disappears while waiting for the other to have an oppertunity to catch up).
'Always up' full duplex comms works very well. Although I also have it connected to my sat nav and phone, when I'm riding in a pair we will open the comms between the two of us and leave them open for the whole day. It's full duplex so you can talk and hear at the same time. At low speeds the clarity is excellent, so it's just like chatting to your mate face to face.
When I'm on my own I'll turn on my sat nav Bluetooth so I get the speed camera alerts delivered to my ears
The auto audio switching doesn't work very well. In theory you can set it to listen for audio sources and automatically change. In reality it doesn't work particularly well as there is always a delay while it opens up the channel and things like sat nav instructions will interrupt in the middle of a sentence from your mate! We find it much better to have the comms open all the time and not allow the sat nav to interrupt.
At high speeds the audio clarity isn't brilliant. Not sure if it because of the noise cancelling, or maybe high volume distortion due to the auto volume, but anything over 70mph and I struggle to understand either the sat nav or my riding buddy.
I have no no idea if the Scala is any better, or worse than any of the others so hopefully you'll get a few more honest reviews on other systems to help you!