Very effective when used a night. They prevent heat lost due to evaporation. I probably keep my pool 5F warmer just due to having my cover on at night.
Being used during the day and getting any heatt advantage is more complex. Some combination of low air temp, low humidity, high wind and having it on will retain heat during the day. But if the air temp is above the pool temp, if the wind is low, and at normal/high humidity (in your climate and mine), it is usually better to leave them off and let the sun do the work of heating the pool.
By the way, it is unfortunate they are called "solar covers" since they tend to reduce how much the sun is able to heat the bulk pool water...although as mentioned in the "during the day" comments above, in certain instances they may have a small advantage.