Has anyone out there used any of the off brand batteries? If so what are your thoughts?

Hi there. Thank you for reaching out. We do not recommend using a 3rd party batteries to run your DJI Product. Using this type of battery might give you unknown issues as we cannot guarantee the stability of this 3rd party battery on your DJI Mavic 2 Pro. If you are looking for a battery on your unit, please access the link below for you to purchase the original battery of the DJI Mavic 2 Pro. Thank you for your understanding and support.

https://store.dji.com/product/mavic-2-intelligent-flight-battery

Hi Joe, I have 3 third party batteries for my original Mavic Pro, with no problems whatsoever, they have everything the original batteries have and fly the same amount of time, however the price of them now seems to be almost the same as original dji batteries, at least for my mavic Pro, a paid £45 per battery compared to £85 for original ones but that was 2/3 years ago, my 3 are still working flawlessly.

If the drone is out of its warranty period, it may be worth considering. Personally, I wouldn't.

Personally as you can still buy these batteries new from DJI I wouldn't risk it.  

I wouldn't.