I disagree, with this caveat. I do believe that startups must conserve cash and believe that the DIY approach can work well if you have time, some skill or just the sheer desire. I've written articles on how to use relatively cheap services to rev up your PR campaign and keep it going. (See: The Resourceful CEO: PR on a Budget and Public Relations for Entrepreneurs )
However, I have used PR firms in my role as COO or CEO in the past - both actual firms with employees and solopreneurs. Both types performed well. Unlike the author in the article, we did some homework to find the PR firms or took multiple references that we spoke to. It's true. There are many out there who promise little and want high pay. Perhaps these are better suited to work with large corporate firms with larger budgets.
In my last role our PR rep did a superb job in issuing press releases, attracting coverage, getting long articles written on our key personnel (including me!) and our company. Our exposure increased signifcantly in six months and had quadrupled by the end of the one year engagement. That monthly retainer was $2,500 and it was well worth it.
If you go into anything with the idea that it's not worth your time, you will find exactly that. It's called the self-fulfilling prophecy. Entrepreneurs and business owners must keep an open mind. Also, you cannot possible rapidly grow a business if you think you can do everything better yourself. If you find that you can, it means you do a horrible job of determining exactly what you need and knowing how to quantify it and so you are unable to properly delegate. Sorry, but when something happens over and over, it's not them, it's you!