I just heard of it (a friend has to take it) and it seems obvious that it's a product marketed directly to business people. I'd expect that the test isn't worth much because it was made in 1955 and personality theory has changed much since and because most research shows little incremental validity (above intelligence) even for the good and recent personality tests in job assessment.
What impresses me, is that they don't even seem to care much about keeping up the appearance of a scientifically grounded test. When I call up their research database, it doesn't return any articles about the PI. They don't link to any peer-reviewed research from their main page, and the FAQ items about reliability and valditiy just serve to explain the concepts.
When I searched the company's name I found one recent study referencing their coefficients, which were apparently assessed by Perry and Lavori back in 1983. The research was commissioned and not peer-reviewed, so it is not going to hold up very well against accusations of bias. They don't show criterion validity, but some other PDFs on this pi-mgmt site (which I didn't find via the main site). None of those show v. above intelligence.
But they say: "The PI has been investigated in over 400 criterion-related validity studies since 1972, (16 of these studies were conducted in 2003 alone) for a variety of jobs and in a variety of industries, and has been shown to be job-related in all of these studies."
It was job-related! Thank goodness. Can they refer to 400 studies if they were never published anywhere, not even online and be in accordance with standards of truth in advertising? With the MBTI the research is there, it "just" doesn't show that the instrument is good and most business executives don't find out. But the PI practices strike me as even more audacious.