These are cheap Chinese-made units. The Manfrotto equivalent is nearly 3x the price. And it shows.
The product description has recently been changed by the supplier to reflect the fact that there is no threaded bush in the bottom of the unit, just screw-in clamp. Therefore, this unit isn't as secure as the more expensive types that do screw down onto your light stands - it's OK, just always check if you move the stand to a new position.
As regards other buyer's comments about modifying the slot to accept their 430 & 580's; I have a 430 EX II and it fit's fine. Maybe Canon changed the soleplate on the MkII?
And the flash shoe should be fixed relative to the umbrella clamp. This is so your flash points at the centre of the umbrella for maximum reflection / diffusion. If you have a fixed flash gun, you just need to move the umbrella backwards or forwards to set this relation correctly.
I rate these at three stars - they do the job for me, and are cheap. They just not as polished (quite literally actually) as the more costly alternatives.