How much of what Bob Lazar has said - about working on alien UFOs - do you believe and why ?

None at all. ALL claimants of aliens are either morons or scamming liars. Or both.

I don't have much belief in what he's said at all... 

If he had one tiny scrap of physical evidence that could be analyzed in a professional lab and then published in a scientific journal, we would all treat him with respect and he would go down in history as a man with a remarkable discovery.

All that is needed is just one tiny scrap. An alien cigarette butt, or a square of toilet paper that a space boogieman had used, or an empty alien gum wrapper. IS that too much to ask?

Apparently it is. All we have is tall stories, the usual dodgy photos, and general run of the mill rubbish that we expect whenever boogiemen from space and their conveyances are the subject. Without physical evidence this is not a question for the physical sciences. It is a question for the psychological sciences to tell us why people are so gullible, and why they so desperately WANT to believe in naked little chromedomes skimming pointlessly about up in the air.

People have been making nice profits from people's gullibility for centuries. Here is just another example.

Horseshit, they are made right here on earth, probably China.