Honestly, 100 backlinks are nothing. More than likely they are posting to forums or blogs, and then maybe 15% (25% if your lucky) of those are actually sticking. Then upon that, if they are giving you a bunch of PR0-PR1's (the ones that actually stick), then you might as well burn your money.
Forget about wasting your money on a service, just go straight to the source. Go purchase yourself Scrapebox, and then buy a Fiverr gig once per week, and you will see your results jump within slightly on week 2, and then dramatically on week 3.
The key is consistancy. However, by purchasing a cheap service, your still in the same league as everyone else is that wants quick results without actually putting in effort.
SEO success is measure by the effort you are willing to put forth.