Turbo-tip -- that has a nice ring to it! I started using them last year and now have them on all 3 of my boats. They're relatively common on the surf skis where they've proven to increase rudder effectiveness, especially with following seas for broach control.
For me, they add a lot of control riding boat wakes where it is common to get into the 8 - 9 mph range. I've done quite a bit of experimenting with the Hobie rudders and think the winglets are a real asset at any speeds over 6 mph. Negligible drag.
Here are a couple of other views of what they look like: