I haven't implemented it on any high-profile sites, but the few blogs I've turned it on for have seen anywhere between a 95-100% drop in their commentspam count, and the sites <i>seem</i> faster, but we haven't measured, and the traffic generally hasn't been high enough to serve as a real test.<p>Still, it's worth signing up for a free account if you have any sort of self-hosted site or blog.