Well, I can share a very limited experience. I tried them during less than a month with three domain names having some traffic. According to ParksDot's own parking stats, in less than a month, one domain got 785 uniques, the other one 209, the third one 262. Click: 0. Revenue: 0.
To be fair, I should add that I didn't attempt to optimize my domain names during most of the period, but did it for the last week of my testing period - it didn't change anything.
Very limited experience - but I moved the three names yesterday to Bodis, where a handful of domains with less uniques bring some revenue every month.
Once again, I wouldn't pass a judgement based on such a limited experience - jut sharing, as you asked.