There are a lot of companies that provide SEM / PPC and there's a lot of differences.
One of the best ways to determine the best from a long list of good providers is third party verification from the search engines themselves.
Google annotates the best search marketing companies through their Google Adwords Premier SMB Partner program. Unlike a Certified Partner, this is not an accolade you can simply meet requirements and receive. Google selects them based on their past performance, customer satisfaction, ability to operate at scale, etc.
Microsoft has a similar program, called the Microsoft Advertising Authorized Reseller program.
Once selected for the program, the engines then offer products, services and betas not always offered through non-selected partners, such as free clicks and click rebate programs.
In the automotive industry, there are only four companies that have been selected by the search engines with these designations - Dealer.com, Cobalt, ReachLocal and Haystak Digital (formerly Moore & Scarry).
All four of them provide great service. If you watched Jonika's (from Google's) keynote address, she highlighted those four providers and discussed the great lengths Google goes through to vett and select them.
All of them are great providers, each with their own set of benefits. Cobalt and Dealer.com, however, only work with dealers who have Cobalt or Dealer.com sites. ReachLocal and Haystak (my company) work with all dealers.
The main point of differentiation between Haystak and ReachLocal is our exclusive, specific automotive focus. Our software is designed for auto dealers and offers things like dynamic inventory ad generation, bidding based on vehicle age and black book value, etc. In addition, we offer fee-based compensation - so we don't take a percent of your spend.
If you go with any of the four listed here - you'll have a solid provider that's been well researched and vetted by the best in the industry.
This is certainly not to say that other providers don't or won't do a good job.
For information on Haystak, visit
Haystak Digital Marketing - Automotive SEM & Automotive PPC