My experience has been they are over all:
I shoot a 2.8 version versus an F/4 version at say 4 or 5.6, I notice better AF and better over all quality on the shots on the 2.8 version. I think with most zoom lenses, they have most of their issues at the end so a F/4 lens may be fairly equal to the 2.8 at say F/8 or F/11, but more wide open, especially at F/4 the 2.8 has always out performed in my experience.
In many case, I find the build and over all range of the lens to be better. This may be subjective, but in a similar example in the 50s, is the 1.2 really that much better than the 1.4 and the 1.8.
Yeah. It is. Granted, it is an L versus non L as well, but I noticed a huge difference stepping up from the 70-200 F/4 to F/2.8, especially in speed. Some may argue the price, and the F/4 is a solid performer, but when it comes down to it, with almost anything in life, if your desired range is at the edge of a products capability, you are often better off finding the product that slightly to moderately exceeds your need.