have been using it pretty much non-stop, picked it up as soon as the store opened on launch day (11/11) and I have used 285MB's so far. as soon as I get home/to work/anywhere with WiFi, I kick it over to WiFi but it is always on and aside from the usage I am putting on it by browsing, GPS, etc. its pulling in email, google voice texts, facebook, twitter, etc. and I stream ALL of my music through subsonic.
so at this rate of about 300MB/week (will lessen as the novelty wears off and I stop using it ALL DAY) I am consuming less than 1.5GB's a month, the $35/month plan for 3GB should be more than enough for me.
I do occasionally use Slingbox and NBA Game Time to stream video but usually the video I watch on it is saved on the device.
had an iPad (wifi only) and a Verizon MiFi before this and the usage was about the same, I used a little more on the iPad because I was able to stream all of my videos on my home server with AirVideo and there is no such app for Android (yet) but so far I am trending well below the 3GB threshold of the plan I signed up for.
Also I have not used this as a wifi hotspot (which is included for the $35/mo fee) but I rarely use that option so again for the usage I described above I will come in well below 3GB/mo.