said by v35_pilot:
but in general how much of this pipe will go unused?
It depends on what exactly you're doing.
Loading pretty much any non-streaming webpage almost all your bandwidth goes unused. Say you have a ridiculously heavy news article that is 10MB fully downloaded and it takes you 3 minutes to read the article. With a 25mbits connection, it takes approximately 4 seconds to download full speed. 4/180 seconds = .022 or 2.2% utilization during that 3 minutes. Streaming a high quality netflix stream is around 5Mbps. So 5/25 = 20% utilization.
Downloading a large system update or a game is just going to depend on the size of the file and if the sending server can saturate the line. I might be able to get 1-2MBps (that's megabytes) for a large service pack from Microsoft, so if you got similar you'd still have 1/3 your connection unused during that download.