I bought one sail from a local speedsurfer that had telltales. He said their primary use was to show him when he was oversheeting. He also was a good sailor who sometimes made it through most of a season with out blowing a single jibe. For me, the telltales ended up useless because the sails usually got wet within the first few minutes of a session, and rarely dried out again.

I doubt that they'd be much use, even if they didn't get wet and stick to the sail.

When I'm crewing in keel boat races, we're staring at the telltales all the time. But the fore and main sails on a keel boat, or even a dinghy aren't moving around anywhere near as much as a windsurfing sail, where we're sheeting in and out constantly, especially in any chop. On the big boats, it usually takes the telltales quite a few seconds to settle before you can decide if the sail is trimmed appropriately. On a sailboard, I would have made half a dozen slight adjustments "by feel" in the time it would take to read the sets of telltales on the headsail and decide what adjustment to make. The other thing is that you need to look at the telltales on both the leeward (first priority) and windward (second priority) sides to make sure they're streaming properly. On a big boat, someone else is doing the steering, so I don't need to look where we're going at the same time. There's no way I'd be looking at telltales on my race sail while at speed for long enough to read them properly.

No harm in trying though.