[exim] Greylisting - how do you implement?

I'm just wondering how people are greylisting, I've come across a few
methods:

http://www.sigsegv.cx/exim-greylist-2.html
http://www.tldp.org/HOWTO/Spam-Filtering-for-MX/exim-greylisting.html
(the mysql implentation)


Any drawbacks?