Its not free, it costs $20:00. However once the full game is released, you get a $20:00 discount on the full price.
I'm tempted to get it, as our Deathwatch campaign is drawing to a close and I was planning to start Rogue Trader, in July. However the chance to beta test the game could be fun.
It depends if we get a chance to do so between now and 25 August as that is the trial period, but we have about 5 or 6 sessions of Deathwatch left.
Oh that's a shame, didn't get a chance to fully look at it as I posted this at work. Still, $20 isn't bad.