As I understand it there are 8 of them, so each cylinder has a carb for it which allows for better efficiency(sp?). You can tune each carb for each cylinder. Im no expert on them this is just what Ive heard from people who have talked about them. Hopefully someone else will be able to shed more lite on the subject.
Webers saw use in some of the early Shelby cars. Little offers the 'wow' factor of popping the hood on a early Mustang or Shelby and seeing Webers. They can also provide excellent performance. Jim has everything you need.
Webers provide an individual runner to each cylinder, with a minimum of direction changes before hitting the intake valve. This in turn provides pretty much instant throttle response and huge power potential on the top-end.
About a year and a half ago I wrote some notes on comparing a 4V versus Webers . I described some of the benefits and liabilities of Webers, IDA and IDF. New IDAs are listed for $500 each, an intake will run you $300-650, used and new, respectively. Linkage sets are on the order of $175-250. Then you have air cleaners which could run $75-400. Also need to consider the water neck and thermostat. A good deal on a used setup would be $2-3000. Complete, new setups are priced in the $5k range.