People are really exaggerating the significance of this decision for Trump. It doesn't necessarily let him off of anything as some are suggesting. It just kicks the can to a lower court to decide if Trumps bad actions were "official acts." It really depends on those courts as to whether or not Trump will skate. This decision delays his trials though, which is what's bad because now they won't happen until after the election, which is good for Trump.
This decision is far better when it comes to the actions of presidents going forward. Now they'll know they can pull bullshit because as long as the come up with a nonsensical reason for why their acts are official, they don't have to fear reprisal.