The charger plate gets a bit warm. If the phone is in a case (I use an Otterbox Defender) the battery doesn't get too warm. (The receiver warms up a bit, but not enough to shorten the life of the battery.) The maximum temperature before the battery is affected (during charging, it's different during discharging) is 45C (around 113F), and the battery seldom gets that warm during wireless charging.
And the phone has to be sitting on the charger when it's charging, so you can use it as a computer (pressing the screen), you can use wired headphones, speakerphone or a Bluetooth headset - but you can't charge and hold it to your head unless you use a rubber band to hold the phone to the charger and hold the whole thing up to your head.