I like them, but they often make you unable to see inherent flaws in your scene.
I did a scene with some metaballed-function-driven fog, the preview didn't show any hints that Z-ordering was wrong with the ecosystem, and I just had to go into PS for correction.

Still, it's nice to look at (compared to rendering 640 in preview quality ;) )

In This Twilight- My FIRST public poser work in 2 years!
Also the reason why I endorse postwork (:D)