As someone who is involved in the hiring process pretty regularly, I would say about 10% of the resumes we receive come with cover letters, but only a small fraction of those letters make it obvious that they've been tailored to our company. Maybe 3 out of 10 will do this. Giving people the benefit of the doubt, half the cover letters we do receive are not unique to us.
Honestly it's probably better not to send a cover letter than it is to send one you've copied and pasted from the internet or that you're sending in identical copies to 50 employers. We don't count not sending a cover letter against anyone, but a bad one can hurt your chances of a phone call.
In my experience though, a good, unique cover letter will all but guarantee you an in-person interview. We'll do an initial phone screen that is mostly for baseline technical competence (talk about projects, etc, we're not going over algorithms or anything here), then a single panel interview that we try to keep under 1 hour and are successful with the majority of the time.