L glass is QUALITY. Having said that, you should be aware of certain shortcomings one of which is a tendency to back focus. I love my 70-200 L F4, it goes everywhere with me but as long as I remember to focus slightly in front of my subject and stop-down to a 'sweet' F10-13 it produces spot-on results. When I first purchased this lense, I took it to my 'test-centre', a disused railway track, and setup a few (also disused) Marlboro fag packets(!) on the sleepers. At first I was slightly disenchanted with the results and had considered returning the glass to Canon. But, with a serious Death Valley trip in the offing, I decided to take into account this somewhat irritating feature together with the revealing performance results and have not looked back since. So take your time and do some proving at home with the simplest of tests.
Have a look at what Bob Atkins has to say:
http://photo.net/learn/focustest/
....and don't look back, er, forward?