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.
5D2, 5DsR. Zeiss F-Distagon 16/2.8C/Y, Distagon 15/2.8ZE, 21/2.8ZE, 25/2ZE, 35/1.4ZE, Otus 55/1.4ZE, Makroplanar 100/2ZE. Canon MPE 65/2.8, 180/3.5Macro, 300/2.8 IS, TC1.4x III, MT 24EX, 580 EXII. RRS TVS34L & BH55, Stackshot. Arca 4x5". Zeiss DV20, Axioskop2+, Axiocam HRc, EVO40XVP.