This is what attends you buying the Planescape box set: infinite worlds, extremely original landscapes, strange "planar" way of talking and much more. If you have played the classical AD&D GreyHawk, Krynn or Forgotten Realms and at the end got kind of tired of the usual feudal dragon-slaying stuff this is what you need.
Inside the box you will find 4 maps (the only thing I didn't particularly like, cause they're not quite detailed) and 4 books which contain: 1) A guide to all the planes (Baator, Mount Celestia, Mechanus...etc), where the powers (i.e. the different pantheon gods) dwell; 2)The planar races your PC's can play (bariaurs, tieflings, githzerai) other than the normal prime races (human, elf, etc); 3)A guide to Sigil, a city that is located in the heart of the planes, known as the "city of doors" as it is full of planar portals that can zip your party from the city to whichever plane your DM wants to send you to; 4)A guide to Factions in Sigil(factions are kind of roleplaying kits that will characterize your PCs); 5)A Monster appendix regarding planar fiends and friends; 6)A couple of adventures to work out the beginning of your planar campaign; 7)A glossary regarding the strange slang planars use (words like: primer, clueless, berk, deader); 8) Thorough explanation of how the multiverse works and much more.
The best thing of this setting is that you will finally treat with what you used to see in your usual "primer" campaigns as godly or "too powerful", this will be a boost of originality for long time players and DM's. The setting is very "role playing", its not a just a normal hack'n'slash dungeon slaying campaign. "Planescape" made me go back to mastering.
"So, primer, you have 2 ways of getting your leatherhead to the planes: go through that portal or wait till somebody puts you on the dead book ...what are you going to do, berk?"
My opinion: buy the box, go through the portal!