I actually have a microdrive cartridge in my coat pocket right now.
Its a re-purposed Psion Quill cartridge that I ought a job lot of when Dixons were clearing the stock. I have a bunch of assembly macro code on them.
I first bought the Wafadrive for the Spectrum as I liked the idea of the extra ports and cheaper cartridges. Got it home and it didn't work; apparently not compatible with version 3 Spectrums, so I went back and bought the microdrives and was amazed at the near instant loading of Ant Attack a fiendishly difficult game to learn to play at the time.
The challenge was of course to spend hours loading programs of cassette to convert to microdrive. Often having to split the program and move sections around using the screen as spare memory as the copy protection methods of the original program would load over every spare byte of memory including the microdrive extensions to the spectrum code.
I upgaded to a QL later and and i think I managed to get the two to talk over the interface one network and inbuilt QL network but I seem to remember they never really played well together.
Oh the memories they were great days, if only the QL didn't have possibly the worst screen memory layout in the world ever which made the under powered, yet beautiful to code, 68008 processor a hard time to do anything grahically entertaining.