Yea, I hear people saying trash about the Windows Phone app all the time, but I've never had an issue with it. I hope Spotify doens't, or Microsoft doesn't for that matter, limit its use on Windows Phone because of the new Xbox Music, I just wish they would team up, everything would be better. Hopefully Spotify updates for Windows Phone 8, I also want an RT app!
I just made a test account for rdio and imported my spotify playlist, just to discover that around 30% of it is not available. The more "alternatives" to spotify I try out, the more I want their wp8 app.
Services I tested so far: Wimp (afaik only available in northern europe and germany): good overall, in some points better than spotify, but you can only login on one device at once (I'm not talking about listening to music!)
Xbox Music: Has more tracks than spotify, bands who don't want their music streamed can sell it too. Organizing playlists is a huge mess though.
Rdio: didn't test much, seems to lack many, many songs