I also like the push-pull of the 100-400 L, it retracts neatly to fit a camera bag. I never understand why the 400 L is described as razor sharp. Canon's MTFs show it not to be in the top league as do slrgear's blur test - it is no sharper in the centre than the 100-400 L. Photozone agrees and its MTF tests have the old 400mm tele macro from Sigma significantly sharper. I can testify to those results having traded a 400mm L for an equally centre sharp 100-400 mm L, and having the Sigma 400mm, which blows both away. I'd buy an upgraded 100-400 if it is really sharp at 400mm as the range is so good for nature photos. For primes, I like the 300mm f/2.8 II with 1.4x and 2x TCs, which give a very good telephoto range for not too much weight.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M 18-55, f/2 22, 11-22