I got the blasck iridium transitions and they apparently go from 42% to 10% tansmision or something like that. They seem fairly clear and I think they'll be ok in fairly low light levels, but not for night riding or anything like that.
I have had black persimmon transition radar and that was good when it was overcast as well, but they were not prescrition lenses (about to sell them on Ebay, but let me know if you're interested – black frame with red bits like this: link
There is a new oakley lense that is supposed to be really clear when not activated, but I cannot see that available as a prescription lense anywhere including the US
Time will tell how much I can use these as year round riding glasses…