I have been using the Canon 100-400mm MkII lens for just over three years now.
I thought it was a bit expensive when I bought it, but I was keen to upgrade from the MkI version because reviews indicated excellent image quality.
I have not been disappointed! This is an excellent lens in my opinion.
If you are a Canon user and are considering buying a versatile medium to long zoom this this lens should be on your list.
See my mini review at Canon 100-400mm L IS MkII Review if you want some more insight.