If your website is large, back-links.ca offers an intense deep linking opportunity at four cents per link, created within hours. Not bad in my experiences.
Seems legit! Guy with 8 posts pimping his service!
Anyways, it's all about a combination of services. Diversifying your backlink building, in terms of anchor text & type of backlinks is critical post-Penguin.
I always recommend a mixture of link pyramids (web 2.0) properties with unique content, several tiers built out, and then high quality links blended in for a nice natural link profile.
As always, ask yourself:
If an SEO company knows what they're doing, why are they selling it...
Unless you're a massive conglomerate the answer is: Because they don't know what they're doing.