From what we can tell, it varied a lot. While the term definitely had some negative connotations, in practice it didn't necessarily imply a bad sort of people - just people who weren't Greeks. The term allowed the Greeks to define themselves as a group by designating everyone else as a distinct other, marked by their incomprehensible language (the probable origin of the word barbaros is the way the Greeks mimicked the sound of other languages: bar-bar-bar). Others were, of course, not as great as Greeks. But that didn't mean they didn't have anything to offer.

There's a lot of ways in which this ambiguous attitude is expressed. One example is Classical Greek historical accounts. Herodotos, who wrote the history of "the conflict between the Greeks and the barbarians" (Persians), liked to play up the cruelty and despotic rule of the Persian King, and the Persians' lack of respect for Greek laws and traditions. On the other hand, he was clearly in awe of Persian achievements in engineering and logistics, giving a detailed account of the Royal Road, the pontoon bridge the Persians built across the Hellespont, and the canal they dug through Athos. Similarly, Xenophon liked to portray the Persian elite as weak, pudgy and soft, led astray by a life of excessive luxury. On the other hand, he admired the character of his employer Cyrus the Younger and the efficiency with which his autocratic position allowed him to rule and foster his lands. He was also clearly very impressed with Persian gardens, paradeisoi, from which we get our word "paradise".

Another example is cultural interaction. In Athens, after the Greek victory in the Persian Wars, Persian barbarians were increasingly stereotyped as weak, cowardly and effeminate, the natural subordinate of the manly Greeks. They were mocked for their dress (with trousers being the quintessentially barbarian garment that no honest Greek would ever wear) and for their docile enslavement to the Great King. On the other hand, Margaret Miller has demonstrated (Athens and Persia in the Fifth Century BC: a Study in Cultural Receptivity (1997)) that the exact same period saw a huge influence of Persian art and noble habits on the lifestyle of the Athenian elite. Something similar can be said about increasing interactions with the Thracians in what is now Northern Greece. Barbarians were silly, funny-looking foreigners, but they were also useful, powerful, and often an example to fashionable rich kids and political philosophers alike.