This is a tough call. I owned an agency for nearly two decades, and I still can't tell you that agencies can provide value to everyone. What I can tell you is that an agency will give you an outside perspective on the best ways to position your company and clue you into opportunities and issues that you might not recognize yourself because you're just too close to your own company.
The problem with agencies—especially small agencies—is that they are often so hungry for work, they play it too safe to maintain an account rather than do what might be needed to reach the biggest possible audience. That being said, there can be some value in hiring an agency to handle your marketing for you:
Perhaps the biggest reason to use an agency is that in any type of business, online or off, you're usually best off delegating work to people who specialize in certain areas while you focus on what you do best: running your business.
Of course, the downside is that an agency will cost you money. As mentioned, you should expect to spend some money to market your business. But an agency will cost you additional dollars over and above the cost of any media you're going to buy. There will be a cost for their creative concepts, project management, and writing, design, or production service they provide. And of course, they'll want to mark up the media charges.
So it's up to you, and your budget, whether you'll want to go the agency route. Personally, because I'm not one who believes in skimping when it comes to launching a business (running on a shoe-string budget, yes, but skimping and cutting corners, no—there's a difference), I would suggest hiring a small agency to help you with your marketing program. But it has to be a route you're comfortable with.