When they go bad, what kind of symptoms might you expect? Anyone have one for a '90 Legacy?

most likely intermittent starting problems...or a no start.....usually you can pick one or two up at the Junkyard for relativily cheap....

I'd expect a CEL also.

they wont always set a cel because of failing sensor. a CEL comes on after 3 to 5 consecutive trips. Which means if the sensor iddnt short out on 2 of the 3 trips it wont set a code. Hard starting is a symptom of bad cam sensor. Grab one off a car in a JY.. Cam sensors fit in ur pocket.. just bring a 10 mm with u and tell them u are "looking" for parts