Sadly, it puts a used car salesman-like stain on the legitimate side of the industry. I'm struggling to recall any instances on here where people recommended their amazing SEO guy and I didn't find that they were not getting close to their money's worth (or often, anything at all).
An easy warning sign is if a company is screaming "we'll get you to the top of search results!". There are no guarantees to this. If they guarantee it, read their fine print...
"Oh, you wanted to rank #1 in NYC for 'lawn mowing'? Sorry, but we guaranteed we'd get you to #1 for the keyword phrase 'lawn mowing with hamster-powered organic mowers' and we've still got 4 months on this here $149.95/mth contract."
It does take work, it does take time, especially if you are in a competitive market, and it does take dollars to pay someone to legitimately do so.
I know lawn & landscape marketing, branding, and web strategy. I'm not anonymous, I just don't self-promote.