Yes, frequently, especially attached to intelligent blog posts! For instance, good, well thought-out articles linked here have insightful comments that sometimes people even point out here.
I've seen good discussions in comment threads on various political-themed blogs for my local area. One that springs to mind is the Seattle Transit Blog. They have a good (bad?) number of trolls. However, they don't tend to overwhelm the discussions about the history of regional transit policy and how it relates to municipality X's service imbalance.
