My 70-300 Zoom was fine, no reason to complain, but I decided to make myself a present so I upgraded
Before purchasing such pieces of equipment I read a lot of reviews. One very good site for lens-reviews is photozone.de
Reading their verdict you might understand why I went for this model:
“Every now and then EOS, the goddess of mercy, seems to speak to the Canon lens designers and this time they listened carefully. The Canon EF 70-200mm f/4 USM L IS may well be the very best tele zoom on the market today – it is certainly the best Canon zoom lens tested locally to date.” Afterthe first couple of pictures I can nothing but agree!
Sure, there is a faster 2.8 version of this lens, but it is not only more expensive, substantially bigger and heavier, it also has slighly worse measuring results in all categories.  So, not being a professional indoor-sports-photographer there was no reason for me to go for that one.