> What is the response rate to those postal mails you send? And who do you address them to (do you find the name of the recruiter or just address it to the general company)

I don't do this often, so I'd be wary of reading in too much. In each case, I got an interview and one job, though that took awhile. (The hiring manager did not comment on the letter, but rather on the table at the top—probably because I couldn't find his name.)

I try to find the appropriate person to contact through LinkedIn, ideally a manager in the department of interest; I wrote to a (small) company president once, too. I try to avoid recruiters if I can help it, though my bias against them is perhaps a little uncharitable. The goals are two: to sidestep the usual application process through a little elbow grease and to differentiate myself in the candidate pool. I'm probably a pretty average programmer, and this has been a useful tool for me.

In the past, cover letters were usually a weak point because I would use boilerplate language that comes across as a bit canned. I've had the luxury lately of not having to do this: the letters were all written for companies I thought I really wanted to work for, so the interest was genuine and (hopefully) came across that way.