I'm not sure if this has been posted before but I can't find it online.
Apparently, if you renew your existing contract (12 or 24 months) with Vodafone for another 24 months, you get an extra gig of data for free each month! I found this out when I signed up for the HTC Sensation on the $29 a month cap. It seems to be an automatic thing but it's best to ask the sales representative IN STORE.
Vodafone's not great in most areas but where I am and on the new 850mHz network, everything is pretty fast.
I don't know about you but I can't seem to even use up 500mb of my quota. The network is still too painfully slow.