Seems a good deal if you want to have effectively free internet access (they actually pay you 33p for it!) on the go while away from home over the festive period. In order to receive cashback for PAYG Mobile Broadband you must top up with £15 within the first 30 days to receive your cashback. The dongle gives you 3GB + unlimited Wi-Fi through partner hotspots for 30 days.
I appreciate cashback is required for this deal, but I've never had any genuine transaction via Quidco declined in over three years and O2 aren't a fly-by-night company so there shouldn't be any problem with them even if a query has to be raised. Never used Topcashback, but as pet2000 says, they have the same deal and have no £5 annual fee.