I've not actually watched any ads so far, all have run in the background.
No pop ups, sometimes there is a button to click at the end of the vid but this doesn't seem to time out (20 minutes away from my machine)
There have been 20 second vids all the way up to 10 minute ones.
I 'earned' 150mb yesterday and about the same again today so have 1/4gig of data allowance, for the time I use it I reckon that 4 days 'viewing' a month should do me.
It's a SIM so you need a 3g tablet, dongle, phone or a laptop with a 3g card in it.
My thinkpad has a 3g module built in so I'll just slap the sim in and leave it, if I had a tablet or get one in the future then I'll stick it in that.