Due to the steady increase of detail in levels, also the role of the level-designer is changing and handled differently in each company. The level-designers at Crytek for example focus on general game-design and the design of the missions, but are not involved in the creation of objects, textures etc. It's a cooperation between the design- and art department.
I know Valve did it this way during the development of Half-Life 2 as well, on the other hand Troika (concluding from what I have seen from Zaphod) let level-designers make their own content for the levels (the interesting aspect here is that both games use the same engine, but the development process is handled quite differently).
What about Gearbox, IO, Raven and other companies? What would aspiring level-designers here would like to do when they start to work in a professional company? Which way is the best?