I've been in Oaxaca for almost a month now, and so far I have to say I really like the city, the weather is awesome, it has a peaceful vibe going on compared to other places Ive been to in Mexico. The food is pretty awesome, quite different from other parts of the country. They are famous for their chocolates, moles, tyaludas, tamales, and chapulines(grasshoppers). If you venture to other smaller pueblos outside of the city such as Santa Maria del Tule, or Tlocolula, you find amazing markets that serve up tasty tacos de barbacoa(goat meat), and a famous seafood soup cooked with a hot stone called Caldo de piedras. Also, a must see place outside the city that has been mentioned is Hierve el aqua, a bunch of natural rock formations that have created these giant pools of hot spring waters, and falls, and of course Monte Alban, the pre-colombian ruins of the Zapotec civilization, simply incredible.
The city of Oaxaca is filled with amazing architecture, from breathtaking churches to colonial style buildings from the Spanish colonization era. Calle Alcala is especially nice to walk at night, its a street blocked off from traffic that is lined with restaurants, cafes, and a few bars. The zocalo is also a very nice place to hang, always something happening there, even during the weekdays, its a cool place to people watch and flirt with the local girls.
As far as the girls here I would agree with all that has been written about the quality here. Its definitely not a destination if your looking to bang quality. I'd say 95% of the girls (18-25yo) walking the streets are below a 6. Occasionally you'll see some 6's or 7's and maybe an 8. On a positive note, they are very friendly, and approachable. I've been taking some intensive spanish classes, and I have been able to communicate pretty well with my street approaches. Although I find mexicans to use a lot of slang words which throws me off. As far as the nightlife, not much happening here, there are a few bars, and mezcalarias here and there, but they seemed quite empty on the weekends. The Collectivo Central bar on Ave Hidalgo that was recommeded has not been open at all, seems like they are trying to renovate the place.
Overall, Id say Oaxaca is a cool little spot to chill out for awhile, learn some spanish, and enjoy some unique food.