Leading is much more timing consuming and expensive than using modern body fillers. It's also a lot more difficult and you have to take a lot more factors into consideration.
Having said that if done correctly it makes for a repair as good as solid metal and you won't have issues with cracking or pourosity like filler. If it does start rusting quickly after application thats down to bad technique and not neutralizing the acid after the repair is complete.
It is possible to complete a repair without filler to a metal finish, it is also possible to complete a repair using lead alone, it helps to be a skilled, experienced panel beater.
Now, lets talk filler. Modern body fillers are very good and easy to use.
If used correctly they make for a good repair. The problem is cowboys use fillers to bodge over rust, holes etc and therefore the public think filler = evil. THE MAJORITY OF CARS ON THE ROAD HAVE FILLER IN THEM! Yes, even new vehicles sitting on dealership fourcourts! It amuses me that there are a large number of keyboard warriors, educated by the discovery channel (they identify themselves by using the americanism "color sanding") that have their little rant about filler and then praise Boyd Coddington, if you watch American Hot Rod they basically skim the entire vehicle with filler and block it all back!
The thing to remember when tackling body work is that a paint job is only as good as the prep work. A lot of people will do the bodywork and prep themselves and then let a professional put the paint as they believe thats the hardest part. It's not that hard to paint to a reasonable standard, but poor prep work will ruin a good top coat in less than 12 months.
That was all quite wordy so here's some pics of a rusty Jag wing I repaired and leaded.
And a vid of George Barris and Bill Hines lead loading...
[youtube]jQ_85fp1avs[/youtube]
