Stick to the high season if you want to see butterflies July - August. We went in June & saw two. The backdrop is pretty but nothing special. Not recommended for people with walking difficulties.

To be honest, I was a bit disapointed by the butterflies. I expected nice colorfull butterflies and what we saw looked like some kind of moths. Anyway, it is true that there were many many of them, it is interesting to see them, the valley is nice. But do not expect butterflies like from Amazon rainforest (or Schmetterlinghaus in Vienna :-) )

In complement to the previous comment: You are strongly encouraged NOT to disturb the butterflies in any way (it is actually prohibited). Disturbing the butterflies can lead them to death before breeding, as they consume valuable energy which they can not replace, as they cannot eat during that period. The population of the butterflies is already contstantly in decline.

This was a wonderful experience for the time we were here. The natural setting had been well thought out. If walking difficulties are known then you may not get all the way around, although there are limited seats along the trail. The Butterflies tend to be a specific breed and in places they camorflage themselves into the the tree trunks & until they are disturbed one may not notice them. When disturbered there are a mass of fluttering wings in between the trees until they settle again.
When the butterflies lands naturally, if you are quiet & still, they may land on your arm,- great for a photograph. The cost, whilst appreciated for the running of the grounds may put people off, although toilets & cafe's are provided. It is well worth a visit to this beautiful natural setting. Well Done, glad we visited here.