We stayed 'off season' at Stavros Hotel for 5 nights and loved the place. It's clean, great location, right across from the beach, right in the middle of town, close to the port... and when we needed to rent a car for the day, it was at the door in 2 minutes! There's an internet cafe a few doors away. We loved the balcony overlooking the street and beach (I imagine this could be quite different in at a busy time). The owners were extremely friendly, quite accommodationg about holding our luggage and making recommendations for things to see.

Unfortunately the isle of Sifnos is having growing pains. The hotel is nice enough but there was a constant flow of trucks in the street. The harbor is also having growing pains. One day there was an oil spill that stunk up the whole area by the hotel. Some of the island is wonderful and if you get out of Kamares you will still find some lovely places.

Stayed 1 night in Stavros Hotel. The hotel has balconies towards the main street, which can be a very noisy experience if you want to sit there and relax - both due to the local traffic and traffic from the ferries which passes under the balconies.
The hotel is clean and the owners are very helpful - with most things.
We got a small double room and payed 50€ for a night - which in fact - we found very expensive compared to other bigger, better and more quiet places in Kamares, Sifnos - and still within 2 minutes walk from the beach.
Stavros Cafe in the same building has the best breakfast / lunch in Kamares, and the people working in this cafe are just great!

This hotel is right on Kamares beach--jump out the door, cross the street, and hop into the water--or watch the kids from your balcony. The water at Kamares beach is only about knee deep a long way out--perfect for young kids or for throwing a frisbee. The hotel is comfortable and convenient--definitely in the budget hotel catagory--so don't expect luxury accommodations. We stayed in their family room--two bedrooms and a kitchenette. The only drawbacks were it was pretty noisy at night and the AC was a bit weak.