Done right it's an effective way to cut down on what's probably already a ballooning budget. Some SFX dudes get the shitty task of tweaking the individual frames, but you save a shit ton everywhere else and, done right, maybe a fraction of a fraction of a fraction of a percent of your viewing audience will ever notice.

Well, unless you're Michael Bay and people love tearing apart anything you do with as much enthusiasm as Harry Potter fans digging through the books trying to link Neville Longbottom to being "the one" instead of Harry.

It's a bit like using stock footage, only using stuff that you've already shot yourself and already fits your filming style.

Now the horrible product placement in The Island, that's another story. They did a terrible job with that. I think he thought he was shooting it like Minority Report and didn't realize that, unlike Minority Report, perhaps the shit he was including (and the way he was filming it) perhaps/maybe didn't fucking fit. At all. Two polar opposites, those films.