Like any service if I were a merchant and I was looking for a company to help me with SEO I would ask for at least 3 references to contact.
It's no different than trying to find, say, a decent builder - you need to speak to people who have used them and ask to see there work if possible.
I'm not suprised you're shocked Jess, there are a lot of comboys out there who have used "black hat" techniques to good effect for many years and have been able to charge a fair wack because of it. The trouble is, as you say, these practices are unlikely to help in the long term as the search engines are getting wise to it and find better and better ways to penalise sites that use such techniques (in fact they just ignore the technique in their calculation of rank).
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