Ersi Villas is a good pick for those who want to stay in Santorini in an affordable place, while being at walking distance from downtown Fira. No caldera view from the terrasse, but you can get it by walking 4-5mn in a nice area. Bus stop to Oia is at 2mn walking which is also very convenient.
Our host Elias has been very helpful: he picked us up at Fira central bus station and drove us back the last day, he got volcano tour tickets for us, gave us advice on local restaurants, etc... He has been very nice.
Our room was good enough for us. It was clean and the bed was comfortable. We didn't have any particular view, but some rooms have a view on the sea (opposite to the caldera).
There is also a small swimming pool, which can be nice to cool down if the weather is hot (we went in April so it was not the case !).
Overall this is a good value-for-money choice !