Well, I had been wanting a little more reach than my 70-200 f4L IS USM lens could give so I sold it and my EF-s 60mm macro (I really liked that little lens) and bought a new EF 70-300L lens from Adorama.
I replaced my 60mm macro with the EF 100mm USM (non IS, already had one but the ex wife got that one in the seperation...) I thought about getting the 100-400 but I really wanted a nice walk around zoom so the 70-300L seemed perfect.
I have spoken to people who have one and they love it. What are your experiances? Good? Bad?
awesome lens, comparable to the fabled 70-200 f/4 IS for image quality (better at the extremes (near 70mm and 200mm), worse (not because it is bad but because the 70-200 f/4 IS becomes like a 135 prime there just about, so it's more really good vs ridiculously good than decent vs great) in the middle when comparing them over 70-200 range and better than the 70-200+1.4x TC for 201-280mm part of the range) (copy variation may swap things around a bit, this is how it was with my copies and I get the feeling with most copies on average but I have seen results that seemed to be reliable where it came out a bit differently)
you give up constant f/4 which isn't ideal for action (it does maintain f/4.5 for a good chunk of the 70-200 range though for stills), but you gain a higher quality 200-300 without the super hassle of swapping TC on and off and maintain a nice travel/walk-around size and weight
it is probably my single most used lens, although the new 24-70 II may bring it to a draw
for pure wildlife focus the 100-400 is better since you get a lot more reach but otherwise the 70-300 has a much nicer range on the wide end on FF for general usage and it's much smaller and lighter (the 100-400, 70-200 2.8, sigma 100-300 4 and such are really different lenses in that they are not also nice little travel lenses too as the 70-200 4 and 70-300s are) and has better image quality over the shared range plus better AF and IS