Lousy. Rob Lyman (A-class) just put on new commercially made "prestretched" forestays last weekend. After an hour they had stretched 4 inches.

This was posted to the A-class website today by Bob Webbon, A-class Marstrom racer:

Vectran works great for trapezes. Hall Spars and Rigging has a standard set that you can purchase. I’ve been using mine for a year and a half, no problems and no apparent chafe and the line seems to hold up well against UV. Some folks have tried composite stays but the minimal weight difference and stretch issues seem to favor wire. Marstrom makes carbon stays, they seem quite durable and sexy but are thicker (more windage) and you really have to keep an eye on the end fittings. I’ve seen some articles where I-14 sailors have been successful with PBO but the cost-benefit ratio doesn’t seem to be there.