Hi Mathew,
In my opinion they don't work. At least if they do work, it is very short term and rely on you to keep using them to maintain rankings.
If i were you I'd avoid this as a tactic and concentrate on content marketing. The idea of creating things that people will want to link to and share. Not only does this help in terms of SEO and improved SERP rankings but it also helps you to build the client's brand. People by from sites they trust and recommend sites they trust to others.
I suppose the most obvious case study that back this theory up is SEOmoz. The only reason I am a paid up member of SEOmoz Pro tools is because I trust the organisation and I trust that this is the best tool to use.
So how did that come about? Well in the early days I was exposed to content produced on this website, things like whiteboard Fridays and expert blog posts (This will depend on the site you are trying to promote obviously). Due to this exposure and built up of trust in SEOmoz, I eventually registered for the free trial, which I then started paying for and still do some 18 months later.
This can be applied to any type of business, within reason. You just need to find what your prospective customers are looking for. Do they want more information about products or how to use them. Do they want to be put in touch with other customers who use the products you sell so that they can exchange tips and advice? (great for UGC)
It my experience, this is not an overnight success strategy but it does work, not only from SERP point of view but also in terms of conversions and brand loyalty. This is a much more integrated approach to SEO, which ultimately is more than SEO - it's digital marketing and I think that is what you should be focusing your efforts on, not PR sites.
I hope this helps.
Elias