99% of our diving is in So Cal (Catalina, Channel Islands, etc.) where the water offers mostly very good vis.
I got one first (very suspect of all the Hype...but I got it on eBay for about $60...so I though WTH). I was very surprised. It definately works. It "restored" (put down the keyboard, all you light frequency scientists...I know its addition by subtraction) a lot of the colors.
Three Tests pushed me over the edge from arm-folded skeptic to enthusiast.
1) The first real test was diving with my wife. I had it, she didn't (call me Guinea Pig.) I was pointing our things she flat out couldn't see - until I pointed them out. The "restored" colors at moderate depths (25 - 40) added enough contrast for me to see more stuff. I'm not buying the 15' arguement. So Cal waters are litterd with Pink and Purple fish, mollosks and algae... you just see them better at ALL depths with this mask.
2) When I did get one for her (they we're GREAT at SeaVision - sent us all 3 - she tried them all on, picked the one that fit best and sent the other two back) she began to see things she passed over. Again, the detail (not just the color...but the contrast, too) enabled her to enjoy her dives more. That's what its all about...enjoying the dive.
3) What color filters do most of the video housings have on the lens.... Uh-huh.
Coming up is kind of weird - I mean the mask definately cuts down on the eye-shock of emerging from the water into the bright of day - I love that. But when you remove the mask, everything is sort of greenish for a few minutes. No biggie, just kind of weird.
I would NOT recommend them for students. One DI told me he won't let students in them because he can't see IN as well - to read their face for panic, conform understanding of commands, etc. Makes sense to me.
Just like Dee, I took a spare mask down on a recent night dive (my first...after 4 seasons of diving with the Sea Vision) just in case. I noticed no problems at depth with the SeaVision. My light cannon blazes, so I turned it off and dove with my AA spare light... I switched off masks - no biggie.
Lastly - Although I'm new to this board, and I'm confident there are a ZILLION threads on Mask Defog, we have simply found no better defog than what came with the SeaVision masks. Nothing we have used (and we've used them all...) has worked as well.
These masks are well made, quality products that actually live up to the mild hype. And its getting around, as you are seeing vermillion/rose/pink lenses emerge from the big guys now.
Enjoy -
Ken