You need to know what is does first. Most devices these days have internal Wi-Fi – be it 802.11 A/B/G/N or AC. And some have no Wi-Fi adaptor at all.
In the first case, the ball shaped adaptor can be used to bring your older notebook or desktop Wi-Fi adaptor up to speed – giving it fast Wi-Fi AC 1900 (2.4GHz 600Mbps and 5GHZ 1300Mbps). Do not worry that it says USB 3.0 – it seemed to work equally well with USB 2.0.
In the second case, you can add very fast Wi-Fi AC 1900 to old your device – again it does not matter about USB versions.
Finally, it is a tad faster because unlike most internal Wi-Fi adaptors that use 1x1 or 2x2 antennas this delivers 3x3 streaming – much faster if your router supports it.
Setup can be a little tricky even if you read the manual – all of its one page. It is actually intuitive if you understand rocket science.
First and most importantly download and intall the driver from D-Links web site.
Then plug in the USB cable to the device and to the computer. The ring LED will flash blue and drivers will be loaded.
Next – and this is also important - go into Network properties and disable the existing Wi-Fi or Ethernet adaptors – the computer cannot have two connections. Alternatively, you can ensure that all connections are set to manual.
Next you have two ways to connect. The first is to push the WPS button on the device and run like the wind to the router and push that WPS button as well. This will allow discovery of the ball by the router. Or simply go to your wireless settings and select a network and enter the password.
If successful, the LED flashing ring will go solid and you will be connected. If you select another Wi-Fi source e.g. a 5, 2.4GHz or guest channel you will need to enter any password for that.
Speed testing is really subjective and depends on distance from the router and type of existing Wi-Fi adaptor you have.
I have the latest HP x360 Spectre that has dual band, AC900, 2x2 Wi-Fi that has a theoretical rate of 867Mbps. What I did find was that using the ball on an N300 2.4KHz single band, and an AC2600 dual band 2.4 GHz and 5GHz router produced approximately the same speeds as the internal Wi-Fi at 3, 10, and 30metres.
The moral of this story is that it will vastly improve earlier Wi-Fi A/B/G/N or add Wi-Fi to a Wi-Fi-less device but it is less necessary for new AC equipped devices. It will also not improve the speed of your existing router – if you have a single band N300 that is all you will get and you should invest in a faster AC one.
I can see a need, it is reasonably priced at A$119.95, and its cute in a Death Star kind of way. I am very pleased that it works with USB 2.0 as well – the majority of older desktops only have that.