I see a lot of sites offering "SEO services" that claim to get you a ton of legitimate backlinks or even raise your ranking to the first page in Google - but these sites somehow cannot even manage to get their sites to the first page or even get a high PR. Why would this be?

Let's take this site for example:
SEO Tools For SEO Experts

It apparently offers an amazing service, but it has a PR of 1.

Is this a reasonable warning sign?
I mean, aside from the fact that it has poor reviews if you search Google.

What are other signs to look for when spotting an illegitimate SEO company? How can we protect ourselves?