Got this from the infamous Erowid website:
Saliva Tests
Are a little more expensive than urine testing, but less than hair or blood. (~$15-$75).
Are considered a relatively unintrusive method of drug testing.
Are becoming more common.
Are easy to administer but require lab processing to ensure accuracy.
Detect use primarily within the past few days.
Can detect more recent use than other testing methods.
Have no nationally accepted standards or cutoff concentrations for detection, making results greatly dependent on the specific product purchased. This could also make results less-reliable and/or acceptable for legal cases.
More reliable for detection of Methamphetamine and Opiates, less reliable for THC or Cannabinoids (2004).