The problem with Webers that most people experience, for street applications anyway, is the transition at around 2,200-2,500 rpm. This often results in a slight hesitation or stumble.
There is a remedy for this, but it isn't much of a problem if the set up is to be used for racing applications, as the car is being run above that range almost all of the time, anyway.
The Webers will increase torque by around 20%, which is good when using a SB. They will increase the value on resale, but one has to remember that some people are wary of buying cars with a multiple carburetion set up if they feel that there is going to be a never ending "tuneability" issue. It cuts both ways.
The Webers are not hard to tune--the important issue is getting the initial set up (jetting, chokes, idles, cam) right for your specific application. To tune them is no harder than doing ignition timing.
As a visual stimulus, they are very hard to beat in the looks department. They are so massive on a SB, that it is the one big magnet that people point to when displaying your car.
