Possibly not one of the Scorsese's greatest films but it's got merit. And Leonardo diCaprio and merit. Why I say? While its surprise ending might have seemed like a downer and a damper (it rains a lot in the movie), Scorsese surely did arrive at it artfully. Here's how. Scorsese's no hacky debutant to lure you in with a silly plot twist. And for most part, the end of Shutter Island feels like that - Leonardo diCaprio's a patient - OMG - Leo's not there to investigate the case, OMG - Leo's a freaking patient.
But then I saw bits of it during breakfast and I realised how Scorsese subtly drops the hint that all that is happening is an elaborate setup. Consider the first shot of them on the island, as soon as they meet Shutter Island's head of security, all the security guards next to him cock their guns. Now, there's no reason for them to do so but on second viewing it becomes clear that they are doing so because Leonard diCaprio is mentally unstable, and on top of it armed too. After this, the head of security goes on to take away their hand guns because of 'penitentiary laws'. In the later scenes, it shows the two cops navigating the harsh terrain while being unarmed which is a nice dramatic touch but we know the real reason now.
In the next scene, they both are driving to the actual hospital - a virtuoso crash zoom shot follows them on the road. It looks as if the subjects are being tracked and wait for it, of course they are being tracked. Leo's being 'observed' as you can see.
Finally at the mental asylum/jail's facility as they are about to enter the main building, we see a patient/prisoner making 'Shh' gesture to Leo. What looked like a standard movie trope, in retrospect becomes a warning to the Leo - this patient must have actually recognised Leo as a patient/prisoner of Shutter Island.
Pretty cool, right? And I just saw the first 20 minutes of the film. Imagine how much more I would uncover if I could watch it all.