This past week and a half I have had the opportunity to use the Canon 70-300 4-5.6 L lens.  I found that the lens was very compact, more so than I had expected.   I also found it to be quite sharp.  In compairing it to the 100-400mm by canon, I found it to be sharper than the version 1, but perhaps not quite as sharp as the version 2.  Your definitely left wishing for a few more MM's, but the compact size of this lens really makes it a great travel and walk around lens.  Links to the lens on amazon, as well as brent rents lenses can be found below.

here are some images from the lens.  all shot on a Canon 5d Mark III