We spent 9 nights in Semiramis' lovely house with 7 persons. It's perfectly situated, beautiful view, clean and well organised. Semiramis was a very thougtful hostess, who makes you feel absolutely welcome. She organised that our two rental cars were waiting for us at the quai as the ferrie arrived. Also we hadn't booked our return trip yet, which happened to be on a national holiday. She organised tickets for us.
Serifos is a small island, not too crowdy and with lovely beaches at the end of bumpy dirt roads. Definitely a place to recommend!