Joe,
We had the assert error with the vex sensors in the prior WPILib release, but the current WPILib release no longer asserts. Which is good
However, for some reason our sensors work inconsistently -- most of the time they read 120 or 249 -- but if I put my hand right in front of it, then it will read a few inches.. maybe out to 5 inches or so before it jumps back to 249. I'm hoping it's a wiring problem (it worked at first with the previous version, but then it started exhibiting the 120/249 behavior and I couldn't get it to work correctly), but maybe not.