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If a customer consistently used 10 megabits of bandwidth (combined up & down) 24/7, they'd suck down 3.3 terabytes of data in that time.  That's a lot.

Back when there was an uproar about Comcast's hidden usage capping (if you went over a certain monthly level, you could get warned or even cut off), people were estimating the secret cap at around 90 gigs a month.

All in all, 90 gigabytes is enough to completely max out a 10 megabit line about 45 minutes a day.  But even when you're watching streaming video, you're likely getting a 300-500 kilobit stream. so you're at 3-5% capacity.  A 500 kilobit stream (with overhead) could be watched for 13 hours or so, 7 days a week, and just get under your monthly cap.

If you're viewing web pages.  Even if they were really graphics and media heavy, so they were like a megabyte per page, you'd have to view thousands of pages a day, every single day, to exceed that monthly cap.

You really have to do a lot of P2P sharing, watch multiple streams for hours, or download a LOT of big files to use up just 90 gigs a month.  But if you got a speed that ensured you didn't go over 90 gigabytes a month, it would be around 278 kilobits. 

Would I want a plan that gave me 15 megabit speeds, but a 90 gigabyte transfer cap that I probably wouldn't go over?  Or would I want a plan that limited me to 90 gigabytes a month by slowing me down to 278 kilobit speeds to make sure I couldn't go over it?

All in all, I'd prefer a metered plan where I pay for what I use rather than an unlimited plan where I pay for what I might use.