For years I've been lacking zoom lens in my gear. While I shoot mostly wide angle landscapes, more than once I wished I had decent zoom lens - mostly for wildlife, but also for zooming to distant peaks from hard attained summit in Canadian Rockies.  Finally in September utilizing Canon sale I decided to open the piggy bank and got me the heavy weapon -- 70-300mm-f/4-5-6L.  I spent fair amount of time doing the research -- lens has been around for a long time (>8 years) and I could not find anything bad. Price has not dropped since it came out first time.  Internet is full of various reviews -- for a good one click here

Now that I took it for few shoot sections I can say it does not disappoint.  Images are VERY sharp.  Consider the above -- 300m focal length shot taken from Mount McDonald above Canmore.  Mountain to the right in foreground is Little Sister -- anyone familiar with Bow Valley can probably exclaim "wow" now - feels like you can extend your arm and touch it.  But real winner is distant Mount Sir Douglas -- you can barely see it with naked eye and it looks so close.  For reference consider this wide landscape taken from the same spot with 24mm focal length of my 24-105L lens:

My only complaint is that it is heavy - about 1kg!  If you are a hiker and climb peaks or go on multi-day backpacks, this is an issue.  Other than that I am very happy with my purchase!