Cold formula bottles are so much easier and convenient. I use our fridge filtered water (it's actually slightly colder than luke warm, refrigerated bottles are colder).
At home: takes out the microwave step which is awesome for both Trevin and I when he is crying for food.
Away from home: I can bring water in the bottle and no worries about the temperature!
Trevin started on cooler bottles from the beginning and slowly transitioned to cold bottles by the time he was 3.5-4 weeks (started on formula at 2 weeks). He was young enough he didn't seem to care. He was a lot more laid back about than Brynlie, who screamed at the bottle every time we had to switch formulas. Trevin never seemed to even notice a difference! So luckily his temperament is more flexible.
Bottom picture is actually Brynlie!! :)