DxO labs has tested and reviewed the Canon EOS 100-400 f/4.5-5.6L IS.
From their review:
“Without doubt this lens is a popular model and it’s not difficult to see why. It has pretty good image quality throughout the range.”
Their own title to the review really sums it up “Still a good all-round choice.” It’s not a glowing review, but this has been a strong staple lens for pro photographers for many years. While an updated lens is certainly on the horizon, there is a lot of value left in this quality lens considering the pricier first-party alternatives. Of course, there are some choice third party lenses on the market too.