There have been some complaints about the accuracy of the GPS function on some Samsung phones, but the company assures us that they're resolving the issues and that there's nothing to worry about when it comes to the Epic 4G:
We have tested and validated both Network Assisted (indoor) and Autonomous (outdoor) GPS on the Epic 4G. With regards to Vibrant and Captivate, we are currently testing software updates which will optimise GPS performance. We expect to be able to make the updates available in September and will communicate more information and download instructions in the next few weeks.
I give Samsung credit for acknowledging the issues and trying to get a solution out as soon as possible. [Engadget]