I agree too, the original boxed set still retain much of its value... especially for the full sized uncut map... and a few things that cannot be found elsewere.
The other Atlases are as follow :
- SWMA 2 : it's mostly just a reprint of the World Book and the Inhabitant Book bunched together with a glued spine.
- SWMA 3 : There's a lot of new stuff, but if you have the Emer Box to go along the SWMA 1 box there's not that much new except the master timeline... and that part is available just ask Vroomfogle to point you towards the right direction
[ I'm too lazy to dig out the link ]
- SWMA 4 : now that start to be interesting, as there's a lot of stuff that have greatly evolved... and it is a real atlas, it contains descriptions of places more detailed than any previous atlases... including some stuff about the East.
All in all, if you play in Jaiman or Emer, with players that don't need to know what the big guys are up to, and what is happening in the grand scheme of things you don't need the SWMA4... if you want to send your players to various places on Kulthea, it might be useful to have it.