We've been using Highcharts in our app for about 2 years and are generally pretty happy with it.

The one thing Highcharts doesn't do well is line charts for data series where the x-axis value isn't always increasing (ie data that loops around on the plot). Usually renders ok the first time but zoom breaks completely in our experience.

For us this issue is a big enough pain for our users (who have been stuck with scatter plots as the only working...workaround) that we'll probably move to plotly as soon as we can now that it's open source.