I had one and a Mi 9T.
It was good though I preferred the Mi 9T which looked better and had a bigger battery. Lovely build quality on both
Camera on both is better than the Oneplus
Oneplus' software is much nicer though, faster updates etc and closer to stock android
Things like wifi calling won't work though as the carriers just don't support them. Less choice of cases too.
The Mi 9T Pro is a combo of the other two. Mi 9T looks but with the internals of the Mi 9
I preferred the Mi 9T of the two I had but preferred the Oneplus 7 over them both.
mm-telecom-uk on ebay is a good seller though the Mi store and amazon have good deals from time to time