The twist felt cheap

Agreed, the coming 'twist' was pretty obvious. Telegraphed as hell, I would have been surprised if there wasn't the twist at all. But they laid it on pretty thick. It is a Shame too the performances of the film were good and the plot itself is not bad, but the twist resembled the cliche "it was all a dream" way too much. Also if you figure out the twist by the middle of the film it severally negatively impacts the enjoyment of the film.

It was based on a book so maybe it's not all his fault.

I liked the twist. Especially cause I got it wrong first time I watched the end. Even the twist is a twist.

The ‘twist’ is telegraphed heavily early on and is undeniable once Teddy is wearing patient whites, so people who think they’ve outsmarted the film by ‘getting it’ early are just revealing their slowness and obnoxiousness.

The real revelation is learning the horror of what Andrew experienced which forced him to create ‘Teddy’, the sadness of that, and what the whole tragedy says about mental illness.

Scorsese’s concern is not some poxy ‘twist’, that’s just an entertaining storytelling device used in service of a film which is about something real.

I swear the trailers gave it away

It was nowhere near as clever as Scorsese hoped.