I turned mine on via Nova and while I don't see the notification bar icon or display, I just downloaded 40mb cut the rope 2 from the play store on LTE and wifi and saw it going faster than 10 megabytes/second. It flew.
Afterwards I didn't get any sort of notification. I went to see if download booster was still on, and it was. It seemed to be working in the background based on the speeds I observed. I had a strong LTE signal (in NYC) and wifi is typically fast, but not THAT fast. (Cut the rope 2 downloaded in under 4 seconds)
I did several tests with mine, tracking data usage on WiFi and the cellular network. No data was being routed through the cellular network while it was in use, showing that the core element of download booster wasn't enabled (using both WiFi and cellular data for the download).