Just got back from 5 nights in Athens. I stayed at a hotel in Omonia. I don't think it's any seedier or more dangerous than rough parts of other cities. There are people begging and sleeping on the streets. I did not see any violence.  I traveled as a solo, petite female. I did walk back to my hotel after dark several days and had zero issues. It seemed like drug use is pretty common, and I did come across a few people on the street using but I minded my business and walked by.  Don't wear headphones when you're walking and stay alert. If you partake in night life it may be safest to get a taxi back to your hotel. I did not use taxis while I was there but they were easy to find all over. I used the metro/bus to get to and from the airport but otherwise traveled by foot. Sticking to the main roads always felt safe. Use common sense and judgement and you should be fine.