October will be off-season for tourism in Greece. That could be good or bad depending on what you are looking for. Most island are shutting down and/or closing. However, there'll always be shops, markets and tavernas open for locals which in the end may be a good thing because you'll be dealing with locals rather than pushy, shoving and demanding tourists.
If you like tradition, local flavor, a Greek Experience then it will be fine. If you want partys, nightlife and over-the-top tourism then it may be disappointing.
As a regular traveler to Greece during the off-season I find it a wonderful time to travel there. Lower costs, nice weather and less tourists to get in the way.
If that's what you are looking for then you'll have a great time. If you want to party, get drunk, lay around on the beach and ogling the other beach goers you'll be out of luck.
Travel to Greece off-season is a joy . . . take advantage of the better weather, lower costs, and fewer tourists ruining your holiday!