All agencies are bad. Some are less bad than others.
Agencies have no interest in you or your career whatsoever. They are a business and you, are a means to their fees, nothing more. Unfortunately, agencies are a necessary evil in the modern aviation world where market demands can change rapidly. Agency fees, the chunk they take from your salary, can vary by agency. The Irish mob probably take the most from pilots salaries and will even convince you to thank them for doing it! I have never known an agency F/O to be upgraded, that's not to say it does not happen.
Do a search on this website for specific information on each agency. You will find examples of agencies; unilaterally changing the terms and conditions of a mutually agreed contract, stranding crew abroad, operating under different names from the same location, making absurd salary/package guarantees, convincing pilots to spend money to travel to interviews, false claims of a commuting contract, owing salary to crew, etc. etc.
The AJX contract referenced above, is the exception, however, this is certainly not based on any agency effort, rather, the insistence and monitoring by AJX that the agencies fully comply with their terms and conditions. Carefully consider leaving your airline job to embark on a career adventure which can easily turn into a career nightmare. All the best.