It all comes down to the goals for your business. Speaking generally, if your goal is to be top of mind and raise brand awareness, Impressions are a good metric. If you're looking more from a cost/benefit point of view and are specifically looking to gain and convert leads, you'll really need to be diligent about monitoring your reporting for how many jobs are coming from your tracked numbers/form fills.

In each campaign, Marketing Center will actually calculate your cost per lead. What you can do is compare that number to the cost of a job (and keep in mind creating a great experience can make them a repeat customer, depending on your industry). In other words, you can compare the cost of acquiring that lead vs. the profit made from that job (and potential future ones) to get a sense of your return on that campaign. Hope this helps!!