Is the benefit worth the inconvenience? I've had my email in plain text on my personal site for the last 2 years and haven't noticed an increase in spam (thanks to gmail, I suspect).
My mail server is configured to be deliberately slow - taking a few seconds extra here or there to respond to SMTP commands. This catches out the spammers who just spew traffic at you ignoring the protocol. Checking sender domains actually exist plus the Zen RBL gets almost everything else. The dribble that gets through all that then goes to spamassassin which does a sterling job. (I make it put messages scoring 5-7 points in a special folder I manually examine every now and then to double check its effectiveness.)
About the only problem I have had are bad recruiters who have made zero effort to ensure I am match for their client or vice versa. I even have a "Note to recruiters" prominently displayed on my resume, but of course the bad ones aren't going to bother reading that. I have to maintain a list of their domains to block since that appears to be the only way to stop them. (Currently 45 entries and counting ...)
http://www.rogerbinns.com/blocked_recruiters.txt
This is my "Note to recruiters"
Amen!
My personal threshold is around 100 people. Pass 100, and you quickly dive into Dilbert territory.
That is probably highly correlated with Dunbar's number