What are some Action Replay questions for Emerald Version of Pokemon?

Some common questions for Action Replay are: Where can I buy an Action Replay? How do I use an Action replay? What items and Pokemon can I get off of an Action Replay? Should I use and Action Replay or a Gameshark? Is it easy to use an Action Replay? Do I have to enter in long codes for an Action Replay to give me what I want like I have to on a Codebreaker?