What impresses me about Gil Levin is that he doesn't shirk away from the proposed evidence that argues against his point of view. He faces it directly and then argues logically why such evidence still in fact does not rule out the possibility that his and the other Viking experiments did indeed detect life on Mars.
In contrast, I've found that people arguing against the possibility of life on Mars simply ignore the evidence that this conclusion is invalid. For instance this piece of evidence:

Vikings would have missed life on Mars
Nature Science Update
Tuesday 13 February 2001
"Early Mars lander missions lacked the acuity to sniff out life."
13 February 2001
http://www.nature.com/nsu/010215/010215-11.html

I've found that the anti-Mars life camp simply ignores it! Or worse they sometimes pretend that this research really did not mean to apply to the near surface of Mars!

Any good scientist always keeps in mind the sensitivity level of his instruments AND quotes them in citing his experiments. Gil Levin was doing what any good scientist should have done in the first place: simply quoting the sensitivity level of the GCMS instrument sent to Mars to detect organics.
NOTE WELL: this research indicating the low level of sensitivty of the Viking GCMS to organics was already known to the scientists who produced the instrument. Gil Levin was only quoting their own data.
I would have no problem with anti-Mars life scientists quoting the GCMS results, as long as they included the sensitivity level along with their statements:

"There are no organics on Mars, except possibly the organic level of 15 million microbes per gram."

And in discussing possible Mars sample return missions:

"We have no detected organic molecules on Mars. However, due to the lack of sensitivity of the instruments used, a one kilogram sample returned from Mars could conceivably contain as many as 15 billion unknown Martian microbes."

A lot less convincing when expressed that way isn't it?

Bob Clark

"For a successful technology, reality must take precedence over public
relations, for Nature can not be fooled." --- Richard P. Feynman