The plastic tab on the mount does not stick out enough over the light when you slide on the light and it clicks.  This makes it difficult to remove the light without it scraping up your finger in the process.  Also, my tab split down the middle the first time I took my camera off.  You need to go back to the drawing table on the mount.

I don't own a Fly6 yet but have just recently heard of it and have been checking out numerous reviews with great interest.  Reviews were positive but all seem to agree that the design for the mount could use an overhaul.  This is the only issue that gives me pause; if the unit could be attached somewhere other than the seat post such as on a chain stay, I would be totally sold without hesitation; other than the mount issue, I think its a brilliant solution.  Even without a small seat bag, I'm not sure it would fit; without the seat bag, I would have to wear a back pack all the time to carry the usual necessities for fixing a flat.  Issues with removing the light I can probably live with (though it should be addressed) - just remove the whole unit.