Lobster gloves work quite well into the high 20's. Too hot for anything over 38 degrees. The functional part is loft (air trapped in the insulation) + heat source (two fingers share a space) + wind block. The lowest I have gone is 25 degrees when mountain biking. They were just fine, my lungs were not!
Keep in mind all bodies are different. My wife uses lobster gloves with heat pack inserted if we go for a walk in 30 degree temps, whereas I usually wear fingered gloves for that activity.
Note: I have poor circulation in right middle finger, standard fingered gloves cause that finger to go numb when riding in temps below 35 degrees.