Paired with my Samsung Galaxy S II, this cable fits the phone firmly. The plastic mould and cable itself look and feel high quality.
I've tried it with a Flash-based USB Mass Storage device (4GB, formatted as FAT32 as is conventional), upon insertion the phone displays a "USB device connected" notification, mounts the drive at /sdcard/usbStorage/sda and runs the MediaScanner to update your library databases.
In my tests so far the USB host controller is quite fast - sustaining 11MB/s read from my USB Flash disk. No doubt it is my flash disk that is the limiting factor.
Also tested with a 20GB external 2.5" HDD (again FAT32) without issue.
Not tried: USB hubs, keyboards, mice, SD card readers, composite device (i.e. 5-in-1 card readers).