Nokia Lumia 638 model is not able to satisfied customers because it used windows operating sytem version 8.1 with 1 gb ram. So it process commands very slowly and surfing of internet is also depends upon this.
The phone has only 5 mega pixel camera that is not suitable for clicking quality pictures. Nokia lumia 638 provides 1800 mph battery in the phone so it gives very less battery backup time while calling or using internet.