I used to love it, but I decided that I didn't want to support it any more due to the unreasonable limits they put on the free users - launching a "bait and switch" service whereby they gave unlimited free use with 10 seconds of ads every 30 mins or something.
Everyone joins what was a very good service, and quite a lot of people started paying more (myself included) both to support non-piracy and to get extra features such as offline playlists/mobile use - then suddenly they pile on x hours a month usage, x plays per track (per month or ever?) and tons of adverts to try to force people to the pay paradigm. I for one refuse to support a company that does **** like that.
That aside, it's a decent service for the money IF you listen to a wide range of music. I found that I used it a lot to discover music, but YouTube does that too. It works well for finding friends' music, but that only matters for a couple of months until you've found all the good bands.
After that, I realised I was listening to the same 100 or so tracks mostly, along with another couple of hundred occasionally. Most of which I already owned. I figured that for £10/month I could buy 15 songs a month off iTunes and wouldn't be held to ransom of having to continue to pay £10/month to listen to it. For my £120 a year, I buy up to 174 new songs and can use YouTube to listen to any others I want to have an occasional listen to, or to try new bands/artists. The earlier stuff was the starting reason, but I think it was the final realisation that my £120 a year was vanishing into thin air that decided it - I don't want to subscribe to music, I want to own it.
So for "not wanting to support them anymore", and "not making financial sense, when I really thought about it" reasons I went back to iTunes. The fact I got a new car with an iPod dock but from which I can't control Spotify might've been a factor, too. If you don't mind a company that uses bait-and-switch, and don't mind not owning your music, it's a decent service with a few issues (updating the client occasionally deletes all the music, for example).