I'm the partner and co-founder a PPC Agency.
The right way is to do it yourself first. But make sure you have conversion tracking, so that you know the basic metrics. For instance, if you are doing PPC for sign-ups, how much are you paying per sign-up.
Then you try to improve it, by either cut Cost per acquisition (CPA), and/or increase your volume of acquisitions.
Increase volume of acquisition is always harder. Most people only know little bit of Search, but Content and Retarget works great too. A lot of product are so new that no body is searching for it, so there are challenges.
You should try this yourself until you hit your bottleneck, e.g. can't increase volume without a much bigger CPA, then you should try find a good agency that can bring your PPC to a whole new level. And it's better doing it now, because you already know your basic metrics, and it will be easy to judge is an agency is doing a good job or not.
Judging by the people who commented there, my feeling is that few of them really had the experience of spending serious PPC money PROFITABLY.
A good ppc program is a pure profit center.
Do it yourself just means that you as founder won't have time for other stuff. Do it in-house means you pay a salary of someone doing it. Agency's cost is most likely lower than a full-time employee, and a good agency knows PPC way more than an individual who had some years of experience.
p.s.
Another point, I happened to work with a lot of start-ups. People seem to assume that only big stupid companies spend serious money on line, small smart start-ups don't. But this is just wrong. Small smart start-ups are spending very aggressively online, because they know they are making profit, vs big stupid companies are often too timid to spend anything. So, a small start-up actually need more PPC expertise than the big companies.