Hi Johnno,
Well aside the quality of streaming, the main improvement is of course is the lack of adverts which are after all are so annoying in every aspect of life. It really is like having your own massive jukebox. The user interace I think is the best out there. I stopped using Itunes because of its lack of adabtability and its dodgy interaction with other programs on my computer. I found Itunes so aggresive on all the computers I've installed it on that in the end I had had enough.
Any way back to Spotify. The quality of audio can be improved slightly by using the higher bit rate. I found, like John, that it opens up the music and I found I get a better speration of instruments. This, used in conjunction with a reasonable DAC can lead to a not half bad music source. I still do use my CD player for certain night time listening sessions when I havn't got the whole world and their mother making noise! Because in my opinion you could have the best system in the world but that is no good if you are being twated on the head with a plastic trunchon by a rapant three year old saying he is hungry every 10 mins.
So Spotify for day, night and reference listening and CD for those occasions that I have to myself.
I decided like you to give the premium service a go. It is a no brainer for the risk of a tenner: and I am still using it 5 months down the line. I think it is good to support innovative companies. I am interested to see where Spotify will improve their service. I.e improved bit rates? continous play albums,,,,, and whilst I continue to get massive use out of it I will shell out the 9.99. any drop it quality or service for long period of time and I would be off, but at the moment it's tickety boo.
Any way sorry my answer was a bit long winded.
Hope it helps make your mind up.
D