It's website is bizarrely similar to Cog's one, they must have been a template. The apps are also very similar, but Play has a few extra features and consequently a busier interface. It does support replaygain however.
Also for anyone checking out cog, go to the preferences and set the auto-updater to get nightly builds, the stable builds dont really get updated. The nighties are just as stable, but the interface controls are more cleanly laid out.
Cog is nice but unfortunately it doesn't support ReplayGain. (Ditto Vox.) I actually use VLC as an MP3 player for this reason.