Originally posted by Tom S. Something I didn't see mentioned regarding the 300 is that unless you are rock solid, it's a bit hard to get really sharp images in less than sun lit conditions, and the less light, the worse it becomes. That's not a slam against the lens, it's a fact with any longer lenses, any movement is magnified. My 400 is even worse. For shooting in well lit conditions or with a tripod, it will return excellent results.
Therefore, I pretty much concur with 200 for portrait and 300 for wildlife or sports.
Tom, you are spot on with that comment! I first experienced the difficulty shooting the 300 with a 1.4 TC on a grass slope with a Manfrotto carb. fiber tripod, no spikes. The cityscape i was shooting was uniformly soft over the 20 - 30 shots i took. (these shots were all taken with 2 sec delays) Went back a few days later, forced the legs by hand into the slope, hung a heavy bag from the tripod, and the shots were magically sharp. As you say, my SMC 400 has the same problem. Later on, got a heavier aluminum tripod, giottos, with large spikes. much better to shoot from, but you can see the 300 and 400 lenses just sort of quiver around the tripod axis. There's a rotational momentum problem there that one just doesn't see with a smaller length lens.
I doubt that some of the reviewers of the 300 knew enough to properly test a longer lens. I have a friend in our camera club that had problems getting his new 600mm Nikon lens, ($10,000 by his report) to be sharp, i suspect for the same reasons.