We interviewed dozens of SEO companies over a couple of years, and we hired five different SEO companies over than span. One-at-a-time, of course. We had results that were the same but different. For instance, one company might get us on page one for a short time, then we'd disappear altogether.
Another SEO company would charge a whole lot of money, talk a good game, call us all the time, send beautiful, full-color reports with impressive stats, but we'd never get on page one, so the only stat the mattered, the black-and-white stat of how many times the phone rang, was zero.
Another SEO company failed to rank us, but we were getting lots of calls, so we would scratch our head, but we were happy with the calls. Until, that it, one day we noticed that WE had an add at the top of Google! This company was just running ads, which is something we could do ourselves, but we knew that SEO would get free clicks for years if done right, and ads, of course, stopped working if you stopped paying. We didn't hire an ad agency, we hired an SEO agency.