Like Dr Neuro, I sold mine in favor of another lens, in my case a 300 f/2.8isII. And I miss the 300 f/4is! Honestly, there is room in my kit for both 300's. The f/4 is as tall and wide as a 70-200 f/2.8 and MUCH lighter. Therefore, I'd be far more likely to take a 300 with me as a matter of routine. With the bigger, heavier 300 f/2.8 I only take on location it when I know I need it.
My 300 f/4is dated from the 1990's but it's qualities have not diminished over time. Like most owners of this lens will report, it's pin sharp wide open and responds okay to a x1.4 TC. Another rarely mentioned benefit of the 300 f/4 is its ability to focus incredibly close, way closer than the 300 f/2.8. It's almost a semi macro. I used to shoot a lot of food with the f/4, plus interesting portraits.
I did shoot sports with the f/4 and it did OK in strong light. The central reason I dropped $6.5k on a 300 f/2.8 was for action shooting, the blindingly fast AF and the extra stop are compelling. With the x1.4 TC it's still f/4.
The 300 f/4is is probably one of the great sleepers in the EF range. OP, if you're at all uncertain, get a second hand one, and if it's not for you you'll re-sell in a heartbeat and not lose a penny. Damn! I'm getting another one!
-pw