Adobe released Flash Builder 4.. and it's a great tool to be sure. For some developers it's either not worth the cost or they don't want to spend the money.. or don't have the money.
Flash Builder will give you full code hints for built in Adobe classes as well as custom classes. It offers a nice way to debug applications with the debugger and includes a way to profile your applications.
Using the Flex 4 SDK with Eclipse will still give you a way to compile your actionscript and flex projects for free but does not include the ability to debug as easily nor a way to profile your applications.
Both methods will give you nice error messages to lead you to problems in your application. It's just a matter of what you need and what you're willing to pay for.
I just want to let everyone know that my Flex working with Eclipse articles are still very valid for using the Flex 4 SDK with Eclipse. I will see about updating them or maybe adding a new entry altogether with my latest use cases and configuration.