Ok... here's resurrecting an old thread... yes, I actually did it - I bought one of these (as posted by Cbulla) from my lds a week ago. It's a "gadget" "gizmo" "dohicky" etc.. but honestly, it really does work. Overall I really enjoyed my dive more with it on my mask...
Pros:
1)Could see my buddy the entire dive, less than 30 seconds of the entire dive was she out of my view with this thing on... great wide angle.
2)Not expensive (especially if you get them from the link above for only $4)
3)Did not get in the way during S-drills (which we did several of to make sure)
4)Did not block my view any more than the rearview mirror in my car does.
Cons:
1)Makes storing the mask cumbersome
2)Easily dislodged from the mask, which means it could get lost easily
3)Is a perfect example of a rule #6 violation
4)Only helps you keep track of your buddy if your buddy stays on one side consistently
So call me what you will, I liked this little "gadget" and plan on continuing to use it, though I may find another way to attach it more securely and permanently.
Aloha, Tim