I got my Akra for my F10 M5 installed a couple of weeks ago. The right muffler had to be replaced as it was rattling very badly on cold start. Apart from that, i am loving the exhaust. It's quite a lot louder and characterful with the windows down, but not significantly louder with the windows up. However, there is no mistaking the new exhaust as there is a very perceptible difference even with the windows up and the sound is nicely more aggressive. I also noticed that the car seems to rev more freely and more responsive, and pulls harder consequently. That's a nice bonus to me as i wasn't looking for more power, but simply more aural pleasure
Here's a quick pic. I din bother posting a review as i figured many have already done this exhaust.