I’ve not yet used my 300/4L IS USM extensively; it’s a bit large and heavy1 for casually walking around (not to mention it lacks a bit in subtlety), so about half the time I think “gosh, I would kill for some long glass right now,” I don’t have it with me. As far as telephoto versatility goes, it’s much more convenient to drag a 70-200 and converter along, even though one takes a slight hit in IQ.
When I do have it along, though, as a braced or tripod-held lens, I do fine with it for snapping shots of things that aren’t moving. As a relatively inexpensive long prime, I’m quite happy with its image quality and reach. Compared to the long end of the incredibly poor (optically speaking) stabilized 75-300 (first generation, not the much improved second generation), there is no comparison. Wide open the lens is just off sharp with a narrow depth of field, and stopped down just a stop things are pretty much razor sharp across the image. In bright sunlight, it’s a very likable lens.
On the other hand, I’m not yet any good at taking action shots with the lens. Equivalent to 480mm on a crop sensor, and near 700 with a 1.4x converter, I’m discovering hand-holding to be a bit challenging even in good light (to be fair, this lens sports only the first-generation IS technology from Canon; it supports mode I and II, but is only good for two stops). In the 500-800 ISO range in cloud cover, I’m trying to pull down shots with exposures near the reciprocal, and that makes a moving targets a challenge. Additionally, if the subject is only 5-15 meters away, I’m not particularly well practiced at keeping it in frame, and when I do get the shot lined up perfectly, I often blow the focus.
Obviously the technological solution is to only use third and fourth generation IS /2 and /2.8 telephoto glass, so I get more way more exposure latitude and the ability to use cross sensors. Of course, that means at least $5000 per focal length. Not happening!
In reality, it’s just something I need to practice at a lot more. Luckily dogs don’t care if I point a lens at them and take a few hundred shots…
1 Yeah, I know it’s not even as heavy as a 70-200/2.8, but it’s not a “walking around the city” sort of lens.