You really do have to remember to bring the glasses with you. I've forgotten to bring them a few times, while other times they were "incompatible" with the movie projection (doesn't work in IMAX). The combination of those 2 made it even less likely I'd go watch a 3D movie as often as I was hoping for.

Another point to consider is when you are going with someone. It doesn't feel right that you get a better immersion experience just because you have the "fancy 3D eyeglasses". It is unlikely your partner will ever buy a pair, so it makes me feel bad. So I like to share my glasses even just for a little with them. This makes the whole experience less enjoyable for both of us. Me, knowing that I'm seeing a better version of the movie and them wondering what they are missing out on.