Not worth it
I definitely don't relish the thought of being forced to sit through an hour and quarter of mind-numbing, soul-destroying advertisements just to get a measly 500MB of wireless data, and I really wonder who would sign up for that.
There are far better deals. On my Three PAYG SIM I get a month's supply of data (up to 2GB) for a fiver. I'd far sooner cough that up than watch ads.