In my experience for b2b, the impact is very small or inexistent. Twitter seems to have a much broader reach and better results.
In sort of the exact opposite situation as @Dario, we experienced fairly good results in marketing a consumer oriented product that would likely be used during leisure time towards a younger audience on Facebook.
It all depends on who you're trying to reach.
To answer your question in the title - it varies, it can be highly effective or useless, as you can see from some of the responses. It depends on the product/service, but also your marketing goals - new customer acquisition, customer retention, brand management, engagement and research. It also depends on who your fans are - existing customers or facebook users you advertised to to build your fan base (a mix for me). I believe for most companies fan base is initially built from existing loyal customers with hopes of expanding fan base to their fans networks.
I don't look at Facebook fan page as an acquisition channel with immediate results, but rather as a long-term brand and customer management tool. Everyone talks about engagement these days and it seems to imply that engagement improves customer loyalty and retention, and in the long run LTV. It can also be helpful as a feedback loop with your customers. For example, when you post about new features or new blog posts the number of likes or shares you get can tell you what is popular and what is not.
And lets not forget about indirect SEO benefit from all those links coming from a domain with SEOMoz highest domain authority of 100/100. Source: Open Site Explorer
Example: http://www.opensiteexplorer.org/links?site=www.facebook.com%2Fpages%2FTest-Designer%2F267814749902223 - my site's Facebook page has a higher (gasp) page authority than my site
Another SEO benefit is the "likes" you get on your site pages, which in turn boosts their visibility. I got more than 10 likes on each of the last 2 blog posts I promoted on Facebook. Sure 10 likes is not a lot, but the pages existed less than a week.
It depends on the industry and the way they chose to market it on Facebook. You have to engage the users and put them at ease that they're not being marketed to. It can be tricky.