I have one of the Alcohawk Pro Breathalysers, it is extremely accurate as far as personal breathalysers go. Its actually rated better than the Australian Police ones. It is still only a guide though, any reading won't mean shit if you get pulled up and test over.
http://www.halifaxandgeneral.com.au/products.php?id=5000
Halifax and General will also recalibrate them (recommended every 6 months)