I've used Spotify for a while now, got to be at least a couple of years, and for all of that I've been paying for the £4.99 package except for the first month I discovered the service.
Music does disappear, and I wish they'd post something about it rather than having it just "go grey" in your list. Sometimes when my complete playlist is on random it's months until I finally realise something I was listening to hasn't played for ages.
It is also missing the majority of stuff by some of my favourite artists such as Metallica, Lacuna Coil, Rammstein, Red Hot Chili Peppers, and others. Unfortunately for said artists, their lack of appearance on Spotify doesn't make me go out and buy their CDs, it just makes me annoyed they're not on there and determined not to give them any money.
Overall, despite the missing music and artists, I wouldn't be without it. Much preferable to buying CDs, and much more convenient than pirating. Yes, I am a reformed freetard, which will no doubt earn me the ire of some of the more rabid commenters on here, but this is exactly what I was waiting for.
Now all I need is for Netflix or LoveFilm to actually have a decent selection of stuff on their streaming service and I'll subscribe to that to!