Thought this was a great deal, and especially well priced considering the price of new releases digitally are normally higher than retail on Steam. Perhaps this indicates the shift from retail to digital delivery.
You get both games, Bioshock 1 and 2 for £26.99, and Bioshock 1 is immediately downloadable and playable after purchase. Bioshock 2 unlocks after 3 weeks. If you already have Bioshock, you can gift it to somebody else.
If you can get 3 other buddies, or you're feeling particularly generous, you can get the 4 pack, which includes 4 copies of Bioshock 1, that you can gift to others, so same as the single deal, but £7 cheaper.