Tiberius' positive effects were that he had a long reign, Tiberius reign was essential for the long term survival of the empire, if he had died suddenly, many would have likely sought a return to the republic, as many people who had lived in the republic were living. By the end of his reign, the last generation of Romans who were raised in the republic had died, meaning that everyone in the empire had lived their whole lives under and emperor, and would have expected a new one after the death of Tiberius. The other major positive I can think of is that Tiberius left the Empire very wealthy, saving money and building up a large reserve of sesteres,(3 Billion I believe). Unfortunately, this would all be thrown away quickly by Caligula. His negative effects are more wifely known, his unpopular and paranoid purges built distrust in the public, and his pseudo-retirement and general indifference is certainly a major flaw in his reign.