T-Mobile USB Mobile Broadband dongle is now reduced to £9.78 + £10 credit (total £19.78) and TopCashback should track at £20, meaning you make a net profit of 22p (or 42p if tracks at the higher rate), get a free dongle and £10 worth of mobile broadband (£2/day, £7/week, £15/mo) on a decent network.
I'm planning to use mine when travelling on a £2/day basis, so I basically get five days free mobile broadband from this deal. There is a 2Gb/month fair use policy. I'd recommend this to anyone looking for a occasional use (i.e.travelling) or as an emergency broadband dongle.
EDIT 2: TopCashBack have dropped the cashback from £20 to £10. It's still a good deal but if you're not looking for cashback the dongle is still available at CarPhoneWarehouse for £1 less. ***CashBackKings are still offering £20 cashback.***
EDIT: I have changed the title to reflect that this offer is potentially net no cost/free, since cashback is never guaranteed. However, you can reduce your impact of the cashback not tracking by sending back the item unused under the DSR or indeed within seven days of receipt under T-Mobile's no quibble refund policy.