How Does Your Brain Process 3-D Movies?

A reader asks Tyler Cowen if we should expect stagnation or continued improvement in action movies:

As for the stagnation issue, there are two main developments. The first is a resurrection of sorts, namely 3-D, which is a very real gain, but in my view it is a significant plus for fewer than ten movies, most notably Avatar.

I'm curious about something, and it's on my mind since I saw Life of Pi in 3-D the other day. Whenever I see a 3-D movie, I notice the depth for about the first five minutes, and then it just goes away. With only occasional exceptions for the most outlandish scenes, I pretty much see it as a flat 2-D movie. How about you?

Please avoid free-form rants about 3-D. I know some people like it and some people don't. I'm just curious about whether my response is common or not. When you see a 3-D movie, are you aware of 3-D throughout the entire film? Or does your brain turn it off after the first few minutes and basically turn it into a flat film?