For most people, you can probably pull a 3G/4G connection of a few megs. Alternatively, if you're somewhere with WiFi (such as at home), you can probably pull a few more megs. But the two are mutually exclusive - that is, if you're using one, you can't be using the other. Or rather, they were - because now, thanks to Super Download, you can run both simultaneously.
Obviously, the app could provide you with some pretty impressive speeds, but it's still in the early beta stages.