We took a recent trip to Naxos and used Blue Star Ferries from Piraeus to Naxos there and back. I booked the tickets several months before so was a bit concerned that there would be problems in picking them up in Greece. However, everything went very smoothly with the whole thing.
Very easy to pick up tickets, easy to board ferry and for us fairly easy to find a seat in economy class which is what we had booked. However, if you want to guarantee a seat it is advisable to pre-book air seats as the ferries get very booked up and there is a mad scramble for the economy seats.
We have no complaints however, were VERY satisfied with the service and would have no hesitation in recommending booking through Greeka.com. The ferry trip just added to the whole pleasurable experience of a wonderful holiday on the beautiful island of Naxos. We are DEFINITELY going back and will DEFINITELY be using Greeka.com again to book the ferry trip.