The only downside about Ios is the 9 hour ferry ride from Athens to get there. There are usually two ferries a day that run to the island (more on cetain lines), one that goes in the morning and one in the evening. This makes your arrival time in Ios evening or some crazy hour of the morning. Luckily, tons of the places to stay there will pick you up at the port no matter what time you get in! It's nice to have a reservation somewhere so that you will be sure to have a seat back in the shuttle bus but even if you don't, there are usually people there who have rooms available just the same! Even though it takes a while to get there, your time in Ios will be well worth it!
Make sure you give yourself a few spare days at the end of your holidays. Greek ferries have a tendancy to go on strike in the middle of summer and although it's nice to get stuck a few more days in the sun, it can be extremely annoying if this makes you miss your flight back.
The only way to visit Ios is to do so by boat. I took the flight to Santorini and then a ferry from there over to Ios. The small island have good ferry connections with other islands as well and also with the mainland. I took the ferry to Athens and back again.
a Bus is good if you just want to head out to the beach..otherwise to go around to Magnari etc...you need a car...I know a friend who missed a bus while coming back from Magnari (there is only one) and had to hitch a ride from a gay couple...:))