I understand that he reigned for 22 years, and that the empire was relatively stable and quiet during those years. However, based on what I've read of his read, his allowing Sejanus to take hold of power so unwittingly, and what seems like horrific debauchery at his villa on Capri, it seems like those later years of his reign would have trumped Caligula's short reign in terms of its negative impact on imperial prestige.
He was. He wasn't deified after his death, which even Claudius was. I just read today a letter from the emperor Julian to his teacher Themistius in which Julian calls Tiberius a tyrant.
I don't quite agree with LegalAction on this one; that he wasn't deified isn't enough 'meat' to suggest that he was a bad emperor, and in the case of Tiberius specifically, he could've been a superb emperor and still not have been deified. Simply because Tiberius himself was very adamant in his own belief (and in trying to get others to feel the same) that he was not as divine as Augustus. As far as it was possible for him, he refused to be seen as a god at all.
So, with that being said, Tiberius isn't on the lists of neither good emperors nor of bad ones. He was fairly average, to put it in non-academical words. He was chosen by Augustus, not because he was the most fitting for the role, but because there were no other options (save for a 12 year old boy named Agrippa Posthumus), but Tiberius didn't want to be emperor. He agreed to become one because it was the will of Augustus and his mother.
As you say yourself, his reign was relatively stable. Tiberius had proven himself to be a good general, so he was liked by the army (albeit, he wasn't a Germanicus or a Marcellus), and he wasn't disliked by the people either. He was also a decent politician, who managed to keep the senate in check and the empire together.
However, towards the end of his reign (and life) he started to get more and more paranoid. Germanicus himself is said to have been poisoned by the governor in Syria, but that the assassination would have been ordered by Tiberius. Furthermore, he supposedly ordered the deaths of the rest of Germanicus' family, except for Gaius (whom you probably know of as Caligula). Caligula was groomed by Tiberius to become emperor after him, but hardly in a pleasant way.
So while most of Tiberius reign would have let him pass by mostly unseen by history, the last years put a black mark on his reign and might trigger some people to remember him as a bad emperor. Personally I don't agree with that. My impression of Tiberius is that he was a rather unlucky person who much would have prefered to be born in another family and eventually been forgotten by history. As it was, he was born to a Claudian and a woman who would eventually become the first lady of Rome.