I have considered the likes of the Tamron 200-500 and Sigma 50-500 and similar in place of the 100-400L I still have as my benchmark. I've never been convinced the extra 100mm is good enough to be worth it.
One time I did dabble with a used copy of the original Canon EF 300mm f/2.8 (no IS) lens. It was old, with a 1987 date code, and it looked it with lots of cosmetic body damage. But that did bring the price down into the ball park of a new 100-400L so I got it to play with for a bit. An actual 600mm lens with AF has never been cheaper.
Here's a direct comparison I did against the 100-400L on a duck that made the local news!:
7D + 100-400L at 400mm at f/8
7D + EF 300mm f/2.8 + Sigma 2x for 600mm at f/8
Both above are 100% crops although I think the forum is shrinking them further. I think they were from camera jpeg so arguably you could process them harder if you use raw.
You could also try using it like a macro...
This is not a crop! Resize only. 7D 600mm f/8.
I didn't keep it in the end. It was too heavy I never wanted to carry it anywhere. Also I felt excessively restricted by the lack of zoom. I suppose if I were to look at this again, the Sigma 120-300 f/2.8 (with 2x on it), with or without OS, could be interesting, but I'm also holding out to see what the 200-400 extender is like, and at what price! Somehow I don't think it'll deliver the "affordable 600mm".