I would suggest renting one for a bit. I was ready to buy a 70-200mm F4L IS and decided to rent one first. I wasn't happy with the bulk and weight (and it's lighter than the F2.8L). I decided to go with the smaller, lighter 135mm F2 prime and a 1.4x extender. Having said that, I haven't bought either yet, so I can't tell you how well my plan worked.