Recently I've been thinking about upgrading my photography kit and trying to decide which lens to buy. Canon is currently running a great special on their professional-level bodies and L-series lenses so there are some great deals right now. It's always important to consider your use-case when purchasing camera equipment. My work covers a broad range of categories but primarilly I shoot sports, nature, and landscape. I'd been debating between upgrading either my 17-40 F/4L or 70-200 F/4L to their 2.8 cousins, or buying a lens in a completely different focal length range.
I use Aperture for all of my photo management, which means that I not only have access to all of my photos but a complete database of all of my photo metadata. I decided to run some analysis on that data to see which lenses that I have used produce the best photos. These results don't necesarily say anything about the quality of these lenses, meerely my own preferences for which lens to use in a given situation and my own satisfaction with the results. Note that I only own two of these lenses, the rest I've merely borrowed.
Out of all of the 5 star images in my library (1300 out of 162655) here is the breakdown per lens:
300mm F/2.8L IS: 305
400mm F/2.8L IS: 151
17-40mm F/4L: 280
70-200 F/2.8L IS: 98
70-200 F4L: 84
The rest are the 600 F/4L IS, the 16-35 F/2.8L, 24-70 F/2.8L, the 28-135 F/3.5-5.6 IS, and point-shoots.
Sadly only three of these are from my iPhone :(
That tells me that the 17-40mm F/4L is by far the best value I've ever gotten out of a lens (600 dollars for hundreds of good shots) and that 300mm is my most used-per-good-shot focal length.
These ratios are about the same if I lower to 1 star images and above (sample size of 25000 photos). The only exception is that there are far fewer 400mm images here (roughly 1800) meaning that the 400mm produces far more exceptional results per-image-shot (which is to be expected since it costs about $8000). I would love to own a 400mm F/2.8L if I could afford one. I think it's the best lens I've ever used.
Overall I think this shows that the 17-40mm F/4L is an incredible lens for the money that has served me well, and that I need to invest in a lens in the 300mm focal length range for my own use. It also shows that my usage of the 70-200 focal length range, even though it's an exceptional range (and both are exceptional lenses) is limited and that my results in that range don't tend to be as good as those in the wider range (landscape, etc.) and at the farther end of the range (closer to action). I'll continue to rely on my 70-200 F/4L for mid range shots, and probably invest in a 300mm F/4L IS for future long range needs.