If you run an e-commerce site, especially one with "cool" or interesting products, Pinterest is fantastic. If you run a blog with interesting imagery like awesome photography, hot chicks, funny stuff, etc., then Pinterest is fantastic.
If you are trying to do web marketing for a local plumber, Pinterest will suck. If you are trying to generate health care leads, Pinterest will suck.
Really depends on what you're looking to do. Pinterest is a good equivalent to Facebook in that people are there to have a good time and spend time looking at cool stuff. If your subject is staid or boring then it's not really a good fit. Some companies have Pinterest feeds just to do social and other than generating crap traffic it doesn't do much for them.
It is interesting to note that SEOmoz shows Pinterest as having a very high value link when adding your site address, though you can't control the anchor (it's a little globe), which is probably why so many companies have profiles.