[jbooth]

A few years ago at least, they were regarded as better-than-most which is definitely not to say perfect.

A lot of mid-sized ad networks and ad buyers actually have a negative incentive when it comes to detecting fraud -- those fraudulent clickthroughs look great on the aggregate metrics, so if you're the only person flagging them, you look like you're getting your customers a lower CTR than the competition, unless your customers are sufficiently educated.