I tested my sister-in-law's lenses a while ago, when I was deciding if I wanted to keep either of them. One is a 100-300mm and the other is a 35-80mm. I think I've decided to hang on to the 35-80 and let my husband sell the 100-300 with the Canon film camera.
Here are crops from larger photos taken with the 35-80mm:
At 50mm, ISO 1600(!) on my 350D:
This next one is also at 50mm, ISO 1600:
Remember, these are crops.
Here's one more, at 80mm, ISO 1600:
I think the problem with the 100-300mm is that it doesn't have IS. Also, I just don't feel the need for those focal lengths often. I was out using my 85mm today at a parade, and if anything, I wanted a wider angle, not a longer one! At some point, you start to feel like a sniper with longer focal lengths. 85mm might be the longest I go on crop. When I finally go full frame, I might be able to convince myself that the legendary 135mm is necessary, and that'd probably be the longest I'd go on FF.
What I need to do is go out with the zoom(s) at 35mm, and then at 24mm, and see how those focal lengths feel on a crop cam. I often need to see wider than my 50', and I'll likely decide between the 35L and the 24L, keeping in mind my hope/plan to go FF at some point (24mm on FF = 15mm on crop, and 35mm on FF = 22mm on crop). I would just get the 24L, since it would be much wider on my crop for now, but I don't know if it would be too wide on FF. Can't test 15mm on my crop since my widest focal length is 18. And apparently, when you get wide, a few mm make a big difference. I have some time to make this decision since I can't see myself making another lens purchase for several months at least.
And all this, after my post about how gear doesn't matter! In my defense though, since I've decided to go for essentially an all-primes kit, focal lengths are big decisions, and if you need a different one, that means a new lens. I'm thinking 24' - 50' - 85'. Two down, only one to go :) Then when I go FF, I'll maybe sell the 50' and get the 135'.