I have the 70-300 IS and it does a pretty good job. I have lusted after a 70-200 f4 is but would want the 1.4 converter as well which although giving 280mm it at f5.6. So then you have the same range, the same f stop at the long end for what ... maybe a very marginal improvement in image quality.
When you can take shots like
<<<this>>> or
<<<this>>> I decided that the £1200 was better in my pocket