The Foundry is a seemingly unending source of content, but I hardly ever use it. Levelling up for the first time, I mainly wanted to see the developer-created content, and it awarded more than enough experience that I never had to look for additional XP sources. Being at max level now, I want to do content that continues to be rewarding and makes my character even stronger, which means that the Foundry falls short once again. (During the last Foundry mission I did, the reward chest at the end was actually completely empty.)
While Perfect World has made an effort to integrate the Foundry into the world, with an (unrewarding) daily quest and NPCs that guide you towards it on nearly every map, I'm not sure whether it has truly found its niche. Using other players' ratings as a guideline, you can easily avoid the truly terrible creations, but the catalogue still makes it quite hard to find missions of a particular type that you like, even with the addition of tags. And style and tone of the player-created content can be all over the place, thereby making a lot of the stories feel disconnected from the rest of the game. Still, I may eventually try to see how the Foundry holds up as a source of alternate levelling content for alts of different classes, now that I've seen all the "official" stories.