I use between 200 and 500GB a month.
So, Dave in Melbourne, Australia, listening to 7 audio podcasts, watching 4 video podcasts, buying music from Itunes and connecting 2 laptops + 1 wii to the internet won't blow away your 5GB limit? Of course not, that's not enough to it. You are a low Internet consumer.
I listen to 12 audio podcasts with high quality audio (3 of them are daily, 9 of them, weekly - average of 25 MB per file) and 16 video podcasts with medium quality format (5 of them are daily, 11 weekly or so - average of 50 MB per file). So let me do the math, considering that none of them are updated in weekends.
3 X 25 MB X 20 (days) = 1500 MB per month
9 X 25 MB X 4 (weeks) = 900 MB per month
Total = 2400 per month
5 X 50 MB X 20 (days) = 5000 MB per month
11 X 50 MB X 4 (weeks) = 2200 MB per month
Total = 7200
Total at all = 9600 MB, or 9,6 GB.
And I don't think that I'm a exception. I listen and watch podcasts that a majority of people also do, like Webbalert, Geekbrief, Tekzila, Twit, Diggnation, Buzz Report, BoingBoing TV, Mahalo Daily and a lot more that you can see - and use to subscribe if you want to - in this beautiful OPML file exported via iTunes: http://futilidadepublica.semjuizo.com/Podcasts.opml.
That's just half of the 20GB from that guy (I can't remember who were), but there's absolutely no way that a 5 GB limit would be enough for a lot of users, because I'm not even counting on the 7 TV Shows episodes that I 'legally' download from (undisclosed location), with an average of 700 MB.