It was not very good.
I am overall a fan of Sanderson and all the work he puts out, and I'm looking forward to his new Stormlight book a lot, but Steelheart was underwhelming at best and actively bad at worst. His writing is never his strongest point as an author, but here it's much more noticeably weak than in most of his other recent works. While the concept is cool, the plot is too easy to predict and most of the big moments fall flat. I hesitate to say the book is childish because it's a young adult book, but that feels like an excuse. Overall, the book is just kind of there. Only one scene captured my imagination. The rest was dull bordering on obnoxious. I was unable to suspend my disbelief in the many, many places where the plot required it.
Steelheart was published by a different group than his usual people at Tor, I think, so I'm hoping that's the reason why it feels so unpolished, like it could have used another edit or two.