I wrote a little about this here: http://peachshake.com/2011/01/02/engagement-secret-the-autom...

I do it fully automated, but take steps to make it look handmade. I've found it to be incredibly effective, with probably a 30% response rate.

Doing it by hand might be a little nicer, but everything is a balancing act. If I can get 80% of the benefit by automating it, I'm happy with that.