Process servers are an integral part of the legal process.
Court documents for cases and procedures dealing with real estate, small claims, divorce filings, personal protection, and more will often require one involved party to "serve" a second involved party with an official document or notification (e.
g.
subpoena, complaints, summons).
These documents need to be delivered to the involved parties in a timely manner and are often required to be delivered by a disinterested 3rd party.
There are different types of firms that specialize in process serving.
The most common are specialized process serving companies and attorney services (which often contract with a process server company).
Attorney services can be convenient and typically focus on marketing to and dealing with attorneys, directly.
They can be convenient if the attorney also has a need for other services such as court filings.
However, for many private individuals or small attorney firms, going direct to a process server company has a number of advantages:

• More personalized service: Attorney services often deal in volume, using the lowest cost resource that they can for a particular job.
  Often they'll sub-contract as well, so the job has a higher risk of being lost in the shuffle (a particularly bad outcome for a time-sensitive court document).
  Process Server firms are set up to conduct the work themselves and have a direct relationship with the professional server fulfilling the work.
  
• Less Bureaucracy: Once again, a firm specializing in serving documents is usually leaner and more focused.
  They are more likely to take less of their clients' time to get the information needed to get the job done.
  

When evaluating firms to serve your documents, it will be important to determine the following:

• Cost: How are costs arrived at? Are you paying for "attempts" or do you pay a fixed amount for a set result? Also, do you pay mileage costs? Are there pre-determined limits to mileage or attempts.
  
• Timeframe: When will the first attempt be made? How often will subsequent attempts be made? If the party can't be found, how soon will you be informed?
• Communication: Will the firm keep you in the loop as to the progress of the job or will you need to follow up? What is more stressful than feeling that time-sensitive documents are in limbo? If issues arise, such as a bad address, or if the defendant has moved, died or is incarcerated, you need to be notified as soon as possible.
  
• Registered and Bonded: If the firm is serious, all the process servers should be registered and bonded

Finally, get a feel for the firm's professionalism.
How a firm's website looks, how they answer the phone, how they answer your questions will all give you a feel for their professionalism.
Chances are good (though not certain), that if a company pays attention to how they look and come across to prospective clients, they pay attention to the job as well.
Few companies tend to be conscientious only in select areas.
This is not a fact but a good rule of thumb.
Good luck and I hope your legal issues go smoothly.