Hands down one of the greatest plugins out there for WordPress security. I've only just recently started to use it and am already very impressed.
The only drawback I've found (and it's minor) is that the database query can take some time when checking for already blocked visitors to the site/admin. On larger sites this could cripple the server. I'm not sure there is any way around this but for the time being I've disabled the feature.
I use in combination with the Incapsula service to ultimately bulletproof my website.
Thanks for the continued development, keep up the good work.
edit: 10th may: 404 detection causes white screen of death...