Thanks for the balanced opinions so far. While the vote isn't really clear, it seems that there is a slight tendency advising against buying it despite its merits.
I think I am coming to reason... and getting that little GAS relapse under control again.
Even though the lens it is a steal at "only" 710€, these 710€ are probably better put towards what I am really lacking: more reach!
I am currently waiting for the first reviews of the Sigma 150-600 S (the lenstip.com review sure looks promising) and the 100-400L II and hope to be able to pick up either one sometime in spring.
Sounds like a good plan. Another viable plan would have to upgraded the 70-200 f/4 IS to a 70-200 f/2.8 IS II, but it looks like that money would be better spent to get a longer lens.