Sometimes heavy investment does not justify good returns and most of the time you are taking a risk along the way. SEO companies only care about earning money and most of the time the link building works too fast and ended up affecting your site.
Be sure to make shore they are just not spamming your link in one location and that most things are done manually I have found is also the best way. Nothing wrong with buying links or getting someone to do this for you, just find the right person. Always ask for references from past clients and there contact details so you can ring or email them to ask them about the service, on top of them providing you with actually traffic increase or link building stats.