The Canon 100-400 lens is worth the money, because in additon to better optics than most 3rd party lenses, it also has very effective image stabilization, which will get you lots of good photos that you would have missed with some other lenses. I used a Simga 135-400 lens for a year and a half even though I really wanted the Canon 100-400. The Sigma was a good lens, but without IS, you had to be in really good light to get good results. After I upgraded to the Canon, I saw an immediate, huge change in the number of sharp photos I was getting. If you can afford it, I would definitely go for the Canon 100-400.
You asked for example photos. Here are a few below made with the 100-400L lens, and here's a link to one of my galleries where you can find more. In the gallery, look at the exif under the photo to determine which lens was used. Some in this gallery were made with the 300 f2.8:
http://www.pbase.com/tim32225/wild_floridaGreat Blue Heron - 1D Mark IIN & Canon 100-400L IS lens:
Roseatte Spoonbill - 1D Mark IIN & Canon 100-400L IS lens:
Cormorant - 1D Mark IIN & Canon 100-400L IS lens:
Black Vulture - 1D Mark IIN & Canon 100-400L IS lens: