When the Portal video game first introduced the Portal Gun, people were blown away by the concept (even in a video game world). This awesome object has the ability to create a blue portal and an orange portal on different surfaces. A person can travel between these two portals, but the laws of physics still apply.
If the Portal Gun existed in real life, someone would undoubtedly launch themselves into the stratosphere by accident. You'll only continue to pick up speed due to gravity, and there's no good way to stop without a violent collision. While the Portal Gun would be a lot of fun, there are just too many ways that it could cause massive destruction and chaos.