The 70-200 lens range an essential for any kit. Canon is pretty famous for this lens, it is sharp, fast, quiet and not too bulky. Any canon sports or fashion photographer will carry this lens. The USM auto-focus system is excellent and really fast. Canon brought out two versions of this lens one with Image stabilizer and one without. The stabilized version is slightly heavier. The IS version has two modes:
Standard: Corrects movements of a static scene (horizontal plus vertical stabilization)
Panning: Corrects the vertical axis only (regardless of the lens' orientation). This lens contains many elements (23), we have had three all with no problems, however a lot of our clients have reported their lens requiring focus alignment issues requiring servicing. Generally caused by a knock. This lens therefore is not the most robust and should be well protected in your bag. Optically primes are better of course, but this is an amazing lens. We find the colours and sharpness of the 135 F2 and the 200mm F2.8 much better than this lens but a zoom is an extremely convenient compromise. Distortion wise its ok, slight barreling at 70mm