I use haze a lot to simulate distance, like in real life, far away mountains are hazy. I use for for effects, like low fog on ground, but it's not always convincing, so I do like Lingrif and Zack: use flattened spheres with volumetric mats, or I create alpha planes in PS and place them in my scene, so they look like patches of fog, like volumetric spheres, but the render time is a lot quicker. To creates the planes in Photoshop, use the free "cumulus" plug in from Flaming Pear. You can also create volumetric clouds in Terragen, and render that, and use it as alpha planes in Vue, provided you render your clouds against a black sky. Terragen is free for non commercial use at www.planetside.co.uk