There's a massive improvement in this forum, especially on my local board after the merit system was introduced (well, not exactly one second after, maybe months or weeks after that). Not sure how related merit and the decrease of spammers are, as the increasing scam projects might also contribute to that condition.
Loyce has some great data available here:
https://bitcointalk.org/index.php?topic=5032314.0. This is regarding the 1 merit requirement to display a signature, rather than the merit system on a whole, however.
Essentially, after the introduction of the 1 merit requirement, the number of posts per week slowly fell by around 60% and then plateaued around that level. That is a huge reduction in spam. A compounding factor may be that the price was pretty static around $6,000 for most of these weeks, and then fell to $3,000 towards the end, and that may have contributed somewhat to the reduction in number of posts, but there is no denying that using merit to stop signature spamming from newbies made a big impact.