Marc Alhadeff tested the lens and concluded:
The Tamron 70-300 F4.5-56.3 Di III RXD (700 euros) is a very good telephoto lens for a A7c or A7III. On a 24Mpix results it will be excellent at al focal lengths , all that for a light weight (545g) and a small price (700 euros). Performances are better than the Sony FE 70-300mm f/4.5-5.6 G OSS in term of background blur, AF and sharpness for half the price and 33% less weight.
Richard Wong also tested the lens:
And this is Matt Grangers test: