I honestly don't really like Google Play. I'm not saying that it's difficult, but it is more difficult to use than Pandora and Spotify. I usually use Spotify (I love it sooo much), but I'm currently using Pandora as we speak. Google can not catch up. Keep trying, Google!! Haha.
Google Play All Access appears to provide a good product, but at $10 a month, Spotify and Pandora will still have their places in the steaming music market. To be sure, Google is a software giant, but Google+ has proved that it's success is not guaranteed. Many people will be wary of All Access for that reason, and others will avoid it simply because it is a paid service. Pandora's free streaming service may not have as many features, but it does suffice for people who want a simple player. Free will always have a market.
Just like with the Zune (which, let's face it.. Was a FAR better device than the ipod), it came out too late. The market is already cornered by familiar and accessible programs and people tend to show a large degree of product loyalty towards the item (or program) that they started off on.