The actor they selected has the chops. I've never been concerned about a good portrayal of Pennywise. I'm more concerned with the shift in temporal setting. They have done exactly what I speculated they would have to do, i.e. change up the forms Pennywise takes because that generation had a different set of boogeymen. Sadly, I don't think that will be as effective. I honestly think a return to the 50s monsters done in the old world style, rather than the hipster mode would have been far more terrifying that a redo of the 80s monsters. The problem is that the 80s monsters are still in comedy mode. They don't scare us anymore. Freddie Vs. Jason should be ample evidence of that. I hope to be impressed, but I just don't see how they are going to overcome the burnout factor.
I actually thing just enough time has passed for the classics redone right, i.e. with a touch by the King, would have been far more terrifying to today's audiences. Werewolves and vampires that are Twilight superheroes would be unsettling. The stories of these guys as they were told in the old days would be almost new and painful. I suppose that was just a road too far for Hollywood. I hope to be proved wrong and eat some crow. I will gladly do it for a good rendition, but there is just too much being lost, not the least of which is the simultaneous storytelling of the book. Breaking it into two films, one with kids and the other adults (or mostly so) loses that unique magic.
In short, I have absolute faith in the actors they have chosen. They are all good choices. The story was never about Pennywise. He has always been just the situation, the storm on the horizon, the trouble. The story is (and will always be) the Losers. If they get that right, the rest will follow. The problem is catching that is lightning in a bottle without good writing. Hollywood's tendency to focus on the monster rather than the characters will always cause them to fail when it comes to a story like It.