Over a WAN connection full screen updates will be noticeably lagged, but a typical 1-3mbit connection will provide adequate results. You won't be able to operate the computer as fast as if you were sitting at it, and typing a term paper won't be the best experience ever, but it is possible.
Typically the RDP is used to help other people with their computers, or to grab a file you didn't save to your thumb drive before you left the house.