For those concerned about IQ, also consider the Zeiss 100 mm MP. Quite a bit better quality than the Canon 180M, particularly no lateral color. Focussing the Zeiss is faster than the 180M in AF. Price is about the same.
I have the 180M, but only use it as lens of last resort. With a TC 1.4x it gives you a 252 mm lens, and that is usually sufficient to be safe for venomous snakes. Has anybody compared that set-up vs. 300/2.8 with extension rings? One of those things on my to-do list.