I just signed up for Triple-Play with them: TV, Internet and Phone services. Only the phone svc is new to me.
My account says that the 250GB cap is suspended and here is a link to their latest statement on the situation:
Here is the data usage from my account for this month, for 19 days.
Note: Enforcement of the 250GB data consumption threshold is currently suspended.
Note that I live alone, that I don't watch movies or listen to music online, that I do watch YouTube vids, download files and web-pages, etc.
It does not look like I'd break the 250GB barrier anyway, having averaged 3.5 GB/day so far this month or about 102GB/month.
You had better check what your local comcast-xfinity site says about your service area.
I don't think that ET uses all that much data. It is not sending you each frame, like a movie would, but rather instructions-codes to your ET-client as to what to display to you. "What to display" is a simplification, but you get the idea. Instead of sending you a picture-frame of RedBaird turning left, it sends a code for "RedBaird turned x-degrees left" and your ET-client does the display-work.