Good. With 2 or 3 minro tweaks it could be great.

Great that this app exists in Mac OS. Simple suggestions to make it 5 stars:
1)Write the name of the disk being / just been tested to the screen! you can take a screen shot, but what disk did you just test? if you have 3 or 4 drives in a system, you have no idea which one the results are for. Bonus points: integrate a drop down menu with the local disk partition names right into the gui.
2) When the test is running, change the START button to STOP, so that no one has to read the manual to know that you can do that.
3) Add a history feature (for guidance see the internet speedtest.net iOS app) so that one can observe changes over time. “Hey, that disk was super fast 5 months ago, and I think it’s getting slower”. even just having the read/write speeds with the date/time and the disk you ran it on in a list would be extremely helpful. Hell, I’d pay somewhere between $1-4 for that app. surely you could write a tiny xml file someplace and have a little display or txt output feature to export the basic results.

thanks! hope these help.

stoliosoda about Blackmagic Disk Speed Test, v3.0