Successful debt collectors?

[Andrew S.]

I’m curious what people’s experiences are with debt collectors (from the point of view of collecting money owed to you).  Let’s say you obtain a judgement against a former tenant of a few thousand dollars.  Often it is very hard to collect on this judgment, so one option is to turn it over to a pro.  What are the go-to companies for this?  Do they tend to be local, regional, or national?  Do they tend to buy the judgment outright ( for say, 10 cents on the dollar), or do they take the task on contingency?  Or do they actually require upfront payment of some sort?