Do the Greeks really think that the cost of hosting the Olympics ended up being a good idea
No but at the time it seemed good even though even then some people disagreed with it
But it was good in a way that it helped the infrastructure (metro, new airport, etc.), showed the world how Greece has modernized, showed the world Greek culture/history (although most people know ancient Greece anyways)
It was also good because a lot of new stadiums were built and it helped some sports develop that weren't so popular before like rowing, women's water polo, etc.
Greece was never that good at rowing and won a bronze in 2004. Since then we have medaled in Beijing and London and also have more European championships golds than any other nation.
Women's water polo was never big in Greece until the girls won the silver in Athens. Since then, they have become a world power and won the World Championship gold as well as other medals in other completions and also recently golds in World and European junior and youth events the last two years gives them a good chance to win gold in Rio.
Women's basketball was never big but they reached QF in Athens and since then have done pretty well in other competitions.
Other than that, stadiums like softball, field hockey, and beach volleyball have hardly been used. Cycling, equestrian, etc. do get used but it probably isn't worth it since they're not popular anyways in Greece and so much money was lost. And too many indoor stadiums were created and now some of them hardly get used since there were already many indoor stadiums in Athens before 2004, but they thought everything should be brand new and let's spend money to make beautiful venues instead of using old ones
It cost a lot but many of us have good memories from 2004-05. I didn't get the chance to attend the games but between Olympics 2004, Eurocup champions 2004, Eurobasket champions 2005, and Eurovision 2005, those were the "Golden Ages of Modern Greece"
EDIT-I know this is off-topic since its about Sochi, but besides a few cases like rowing, judo, and some women's sports, most sports either stayed the same or declined in Greece after the games. A lot has to do with the Eurocup title because after that football became even bigger than it already was