I used extension tubes with my zoom to get in close. The biggest problem with a place like this is that it is hot and humid inside and the camera and lens fogged over and it was about a hour before I could start shooting. Earlier is better than later. As the day progresses, the butterflies become more active and harder to shoot because they don't stay still so much. But still, what a great opportunity.
Spent about 5 hours at a butterfly garden near Victoria on a trip to Vancouver Island back in 2008. Don't actually know what kind of butterfly this is, but, as you can see, it was somewhat cooperative.