Well, here in Western Washington, strong direct sunlight usually isn't an issue. :-) I actually did make a trip or three in the day and it worked just fine, provided the tint patch on the windshield provided some shade for the VFD. I was actually thinking of playing with some polarizing materials to filter out some light, but never got that far.
I would theorize that in an area of strong sunlight - say Tucson, Arizona in August - the image would get washed out.

I did also get a smaller, pre-built HUD from a company called 'Defi' (http://www.defi-shop.com), which came with a similar but much lighter tint-strip for the windshield. I had it in my Talon for a long time, and it had absolutely no issues in direct sunlight.

Dan (and I really will go change my sig *right now*)