As Joopson already said BSP is mostly used for blocking out and even that is nowadays rather done in a 3d modeling package than inside the editor. But if you look at the maps that epic ships with the UDK you can see that they use BSP sometimes for simple flooring or straight walls.
Besides that im always encounter heavy problems when working with BSP. Whenever i start blocking out a level with pure BSP the editor becomes more and more laggy, so that i have to rebuild the lighting as soon as i make changes to a BSP brush. Im not sure if im the only one that encounter this kind of problems, but you see that it could slow your workflow down rather than speed it up.
And last but not least, meshes will give you way more control and flexibility, like vertex painting, controlable lods, far more details, destructions, poly-reduction and so on.