That's right, for less than a quid/buck you get 3 months of Spotify Premium.
Should you take advantage of the offer and are a cheapskate like myself then I recommend cancelling the sub immediately - you'll still have Premium for 3 months but your sub will expire automatically with no risk of any unwanted charges.
I've never used Spotify before, or any other streaming service, but the library seems impressive. Sound quality is disappointing though. I'm guessing 320kbps at best.
Offer is good for another 10 days I think, and open to new members only.
https://www.spotify.com/uk/premium/