Works well. I always take it with a grain of salt (always checking the person marked). I've used the extension for many years, and have only found a handful of instances where it was obvious as a miss-flag, otherwise seemed pretty accurate. The creator does kinda suck though. Though I don't use Tumblr, so that might be why it's been fairly fine for me. I've heard that it can be pretty hit or miss there as people use it against each other.