You may well use Spotify differently, negating the argument about whether 320kbs is anywhere near lossless quality. Adressing that first though, 320kbs can sound pretty darned good, conversely it's not CD quality (same as lossless FLAC and ALAC).
I use Spotify for background listening, where the sound quality difference is irrelevant. This probably reperesents 50% or more of my listening, so a big plus for Spotify already.
I also use it for trying new music, for which it is fantastic. If I like something I'm straight onto Amazon to find the CD (god bless Zoverstocks). So rather than stop me buying CD's its having the opposite effect, but ensuring I don't make any bad purchases. It's also useful for downloading to your phone / computer for offline listening, as already mentioned.
For me £10 a month is good value. If only they would bring out a CD quality service (for which I'd pay at least double). I have to say the debate about £10 a month for any of the music services makes me smile when you realise how much the general populace is shelling out each month on Sky!