Have this pre-ordered, but can't decide if I want to play the beta or keep it as a totally fresh experience for when it is released in a couple of weeks time. Game looks like so much fun.
I'd strongly suggest playing it. Tactically, you won't get an advantage simply due to no upgrades, only skins. And you can sample heroes. Which people can do when the game comes out.
I'd play early simply as it's just good fun. No point in denying your self that.