I use and love BTunes, but it has some issues on Froyo. It works, but not the same way it did on 2.1. I still use it as my primary player though as I haven't found anything else I like better. It does seem like the app has been abandoned though. I've emailed the developer and got no response from him. Not sure why the app has been abandoned. I'm half tempted to look at making a bTunes clone that will run on 2.2.
MixZing takes some sort of data and sends it off to their server. I have no clue what that data is and when I asked the dev about it, he was basically a prick and wouldn't answer the question. His answer was that they collect data about your library so the server could stay in sync with it. What specific data they collect and why their server needs it isn't stated and he flat out refused to clarify it so I would not recommend it.