IMO unlike regular heat pads I never get the feeling that I’m being “cooked” the heat is more subtle but feels more penetrative if that makes sense ? (Sorry English is not my first language) It does get heat into the tissue though, I have their bigger pad for muscle injuries and am very happy with it as well. Same thing applies, muscles react as one would expect with heat treatment but I never feel uncomfortably hot which is the case with regular heat pads.
The performance from the AA batteries is virtually the same as from mains supply (as far as I can tell anyway) The only difference is that on batteries they recommend not connecting the optional thermostat so it runs at max heat only whilst on batteries. You can use the thermostat with batteries but it shortens the battery life, not that that matters much these days with most people using rechargeable anyway (which is what I do) Heating up is rapid.
Any more questions feel free to ask. I’m not here every day but will answer asap