Location: A bit far from Chengdu, requiring a transfer from high-speed rail to an intercity train. South Bridge is right outside Dujiangyan, with plenty of food and drink options nearby.
Crowds: Even after the National Day holiday, it was still very crowded on a rainy day. Near the Fish Mouth after Anlan Bridge, there was a queue for the sightseeing bus, which costs extra.
Sightseeing Experience: It's better when it's sunny and not raining, offering panoramic views of Dujiangyan from a high vantage point. Rain and fog make it difficult to see anything, so it's not worth the effort. The advantage of rain is the large water flow, which is spectacular. There's a queue for photos at the Dujiangyan Monument; there are no signs indicating this, so I ended up queuing for quite a while thinking I was visiting a tourist attraction.
A leisurely visit might take 2-3 hours; on a rainy day, 1 hour is enough.
In short: Good weather is fine, and light rain is also acceptable, but heavy rain might be a bad time to head home.