It's simply the best.

Link building is the single most effective activity that can be carried out to improve a website's search engine ranking. There are many factors to consider when aquiring links and there really is no substitute for experience in assessing the best strategy to take - if done incorrectly, link building can sometimes actually make it difficult for a site to rank well.

But does is work?

Yes it does. Well, usually.

If the market is really competitive for a particular term ("web-marketing" for example), then it is very difficult to get to the top of search results. Otherwise, it's a pretty strraight forward process. You build a neighborhood of relevant sites that have links back to you (created in a careful way) and there should be no problem significantly improving a site's rankings for particular terms.

How long and how much?

It can take a while. Usually it takes at least a couple of months for a semi-competitive term and sometimes as long as 6 or 9 months for really competitive terms before significant changes are noticed.

In terms of pricing,  a few thousand dollars over the course of 6 months is often enough. If it's a really competitive field, it can cost considerably more because it needs considerably more work.