Initially ( like 30 years ago ) I became under the impression that the holley was a cure all for re-powers and mods to various engines ......they have even been fitted to straight 6's and 4's.....which sorta doesnt seem right .... probly coz they so look the part sitting atop a V ( 4, 6 or 8 ) engine. But then I started hearing about carbs like the Rochester quadrajet and carter thermoquad and the opinion there was that these carbs ,whilst often being removed in favour of a holley were actually a better carb the problem being that not many blokes (or girls ) were capable of tuning them .....and to be fair the quadrajet is a bitch to set up and parts ( here in NZ anyhow ) are the proverbial hen's teeth where as I can get Holley parts and kits fairly easily .
In the states Holleys can be bought all set up ( emissions and everything ) to be retro-fitted to some vehicles , so it makes sense to make life easy in that respect .
I used to think that a set of either side or down draught carbs was the way to go but then I think that there were a lot of nascar engines fitted with the ole 4 bbl holley and those blokes ( and girls
) dont fuck round when it come to power. Thought a bit more and realised that nascar is really about full noise foot thru the firewall type of driving and part throttle or idle to mid and full range transition didnt really come into it all that much......which while sounding like fun aint really much good for road use so I went back to favouring the multiple carbs with one bbl per cylinder and to that effect I have collected 3 weber 44dcnf carbs for the rover v8 ( I need another one ).............ah but then 2nd hand fuel injection set ups became available for reasonable money so I branched off that way ( I have a pile of go fast parts that have been shelved as soon as a better idea showed up ). As Wayne says the holley is way cheaper than a bunch of webers ( or delortos ) and probly easier to get running ok . I cant help thinking that the webers are probly a better way to go IF you can ignore the cost .My Holleys require metering plates to be exchanged to alter fueling rather than just changing jets ( there are kits available to fix this that have metering plates that have screw in jets ). Seems to me that the holley is fairly easy to get running about right but getting them spot on is a bit more difficult.
Then we have the economy issue ...........faaaark , if I had a dollar ( pound
) for every time some clown said " oh yeah but the double , triple or quad webers eat petrol "........which is mostly bollocks BUT from my own experience when one has a well set up set of anything its such fun to bury the boot one tends to do it all the time 
and that sure doesnt help fuel consumption . I have a 900cc Ducati ( well more than one ) that has been tweeked to deliver a mick hair under 100hp and even when caning the arse off it still returns an easy 50mpg and it has a 44dcnf weber ( which is why I started putting together a set up for the rover .......not for economy but more because they are (were ) about second hand reasonably cheap and should deliver the goods ). After all the fucking about as the plan evolved ( quad webers and fuel injection ) I have come back to a 4bbl holley....... because it looks right ........big shinny air cleaner sitting there in the middle of the engine , traditional hot rod stuff. The other reason is that this project has taken so long I have matured from a full noise high end power fucking hooligan behind the wheel to a more relaxed driver more interested in a nice drivable motor ( her in the passenger seat had a bit to do with this evoloution 
so did the feckin police ) A 390 vac sec sure would wake up a essex v6 ........nice. I have a friend that has such a motor in his 13 foot steel hulled jet boat but he has 3 IDA webers on it and it takes about an hours worth of river blasting to get thru 50 litres of juice but fuck its nail biting fun .......them things dont have brakes ya know, and there is nothing soft to bounce off just rocks and overhead willows. There is a sticker next to the passenger grab rail that says " sit down shuttup and hang on " means every word