Ok, to clear this one up...
The Palantirs are not by nature evil and were granted to the Kings of Men as gifts. They became corrupting only when Sauron captured one and those who looked into the Palantirs started to make contact with him. However, they were always quite dangerous items to use as there was great strain involved in their function.
Two things make you more ready to use a Palantir:
1. Your basic level of power
2. Your 'right' to use it.
Saruman and Denthor both qualified. Saruman had the power and Denethor, as the leader of the world of Men (and with some royal blood in his veins as the Stewards were not completely vulgar), had the right even though that right was not as strong as would be for the line of Kings descended from Elendil/Isildur. Also Denethor, in the books, is noted as being quite a powerful Dunadan with the gift of limited foresight (as some Dunadan sometimes have, an off-shoot of their Elvish roots).
As ever in Tolkien's world, power is more evil than right. Sauron was able to twist Saruman's will towards greed and treachery as they linked via the Palantirs. Dentheor, with the right to use them, could not be so corrupted. Instead, however, Sauron showed Denethor glimpses of a hopeless future, and did this over a long period of time to the point where Denethor was driven to paranoid insanity to the point where Denethor thought nothing could be done.
Pippin did not vaguely qualify and so use of the Palantir by him was incredibly hazardous. He could never have been corrupted by it, though, as that would require prolonged contact of which he was not capable.