Pros and cons of visa agents

For long-term (in BKK): – Yearly extensions of permission to stay (based on original Non-Imm O-A visa) – Yearly multiple reentry permits – 90-day reports What are the pros and cons of hiring an agent? For which of the above do you still need to go to Chaeng Wattana, or can the agent do it all? In your experience, approximately what does it cost, and is it worth it? How to find a honest, reliable one? Thank You.