It’s OK.

Works fine with internal SSD but can not show correct speed for my Lacie moblie hard drive (2.5 inch). As other mentioned, it will alwasy show 15 for my external hard drive.
Also the UI is too big for rMBP 13 although you can drag it and the data can’t see dont matter too much.

WildGeog about Blackmagic Disk Speed Test, v2.2