I'd have to agree with yo-yo on conversion quality on spam sites. However, there are advertisers who are more concerned about their image and brand than conversions. These are the same type of people who will bid way up beyond unprofitable ppc levels just to be #1 on the serp.
Its really just dependant on what perspective you are looking at it from. As an advertiser, anything that sends you converting traffic is good. As Google, a mixture of both, some blogger pointed out that google is making way more money from spam than they are paying their algorithm guys to get rid of it. From someone building a quality site, they are bad if they are taking your rankings, but not always; I've gotten tons of traffic from these sites that scraped wikipedia's links. As a user, they are good or bad depending on what you are trying to find.
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