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I would rate it "better than expected". Didn't think I would like it at first, but it turned out pretty alright. I use my own usb stick for mp3's but I do switch it up and turn on slacker from time to time.
Oh, I hadn't even thought about that. I tried using SD cards in my Golf but there was something it didn't like about certain filenames where only about 1/4 of the songs ever showed up. I fought with it for about 4-5 hours, constantly exporting from iTunes, renaming, etc, and finally gave up.
Which USB port do you use? One of the 'phone charging' ones under the cell phone holder panel? Or is there one somewhere else I'm not thinking of? I suppose a stubby little USB stick might work well in the rear seat vent area too.