Press releases should be part of a multi-pronged media strategy. The best approach to getting coverage is building a relationship with a targeted group of journalists who cover your type of product. Unless you have these relationships already, this is best done with a PR firm who has these relationships and can help you tailor your messaging for each outlet/reporter.
However, there are so many outlets and journalists/bloggers in each space that you will need to do some blasts to reach the broader group. Your blasts should tell me very quickly what your product is and why I should get excited about it (do NOT send long, rambling corporate-style press releases). Pictures can help a lot. Like Joon said, journalists get dozens of pitches a day and you have to grab their attention quickly or it will go right into the delete pile. I can't tell you how many BAD pitches I get, where the company is just throwing time and money down the drain. Really think about what that media "elevator pitch" needs to be to get you the coverage you're looking for.