This looks like a very good deal, and one that I would have snapped up ages ago if Three had done such a thing. The above deal is decent for families who use little data, but for £59/month you can get 4 x 15GB SIMs with unlimited texts and calls. In comparison, Three is £21/month for 1 x 8GB SIM with unlimited texts and calls.
I like Three and am loyal, but when contract time is up, a family plan is likely to grab me no matter who offers it.