These sites are quickly becoming something of the past... They usually just spam you around the web, which will actually end up harming your PR rather than increasing it.... The absolute best way to increase your PR is via quality content and utilizing efficient white hat SEO.
Seriously... anyone who boasts that they will increase your Alexa ranking is blowing hot air... Alexa is no where near accurate, ever. They gather their stats by people who have willingly installed the "Alexa Toolbar".. In fact, i know a guy who did a test on Alexa rankings with 2 websites who were garnering a difference of roughly 20,000 views/day...
"Website A was getting 20,000+ unique visitors per day, and Website B was getting just over 1,000+ uniques per day. In other words Website A was getting 20 times the traffic of Website B, yet the Alexa rank had Website B getting double the traffic within their reporting!??" - read his full article here
How Does Alexa Track Traffic – Do They Really Have a Grasp of Your Traffic? - Ways to Avoid SCAMS Online
On to backlinks... The best strategy to build solid backlinks, is by not focusing on them at all, if your content is captivating and "worth while", natural backlinks will build, and you will be much better off for it..... forum posting for backlinks and the like are also becoming outdated at an exponential rate... Long gone are the days when spamming your link around the web counted as a "good backlink"... and that's exactly what the vast majority of these companies do... they spam you around the web, usually to other pages who have nothing in common with your website, which is not a good thing in the eyes of Google.
Now, i'm not talking about a legitimate "SEO company", although that term is almost a joke because of how simple basic SEO strategies are today, because a legitimate company will likely deliver you the results you want, but its the "automatic SEO spam bots" that are going to hurt you. Basically anyone who talks about spamming backlinks and Alexa rankings are not to be trusted, because their techniques are 2+ years outdated.
So... Ultimately what you're paying for is... outdated black hat SEO techniques that will do more harm than good, in the long run.