I needed a zoom lens for my trip to New Mexico and the Canon 100-400mm L is perfect.
Prior to getting this lens I bought and used the Canon 70-200 F4 L IS and fell in love with the L lens series. It is just incredible. There is so much to be said for the Canon line of optics in general, but wow. Even at F5.6 you see incredible light capture and clarity. In New Mexico I took a number of photos of Hummingbirds and I was excited to find how fast focusing this lens was at F5.6. If you look closely you can see the individual strands in the feather hairs of these Ruby Throated Hummingbirds.
Overall I was extremely pleased with the performance and range of this Canon 100-400mm F4.5-5.6 IS L lens. I also happened upon this Elk drinking from a stream that I was fishing for trout in. It felt like I was in a National Geographic moment as I watched and listened to this majestic creature refresh itself. I wish I had bought this lens sooner. I had considered selling it after my New Mexico trip, but now I don't know if I ever will.