I traded my 300 f/4is on a 300 f/2.8is. The 2.8 is absolutely brilliant, but I often wish I'd hung onto the f/4 as well. It was pin sharp wide open, was completely hand holdable & weighed next to nothing compared to the 300 f/2.8 which meant I took it with me far more often. And it focuses much closer than the f/2.8. I shot a lot of food with the 300 f/4is. It looked brilliant.
IMO it's one of the true performance bargains in the Canon L lens lineup and would be difficult to improve on, especially for the price. The current lens has a lot of fans.
I doubt we'll see this lens updated for quite some time.