The best product managers I've worked with have been ex-engineers. I got technical specs that we could go back and forth on. Wonderful in-depth technical conversations.

The worst ones: I wrote the specs. Instead of talking about technical stuff, I have to explain why something like "getting cache coherency right" is going to cost three or four days, and that no, I can't go to another scheduling meeting right now, didn't we have one yesterday?

The best: Shields.

The worst: "Why isn't that bug fixed? I'm going to call a meeting about that bug at 4:00, and I want it fixed by 5:00" [seriously. this happened]