We used it in my previous companies and our developers were very happy about it. Previously we were using http://www.amcharts.com/ good product (but we switched because at that time amcharts was flash only - I think), but my dev colleagues told me that Highcharts was more powerful.
P.S. at that time (4-5 years ago) it was possible to use Amcharts for free in commercial app but with reduced features (for example you couldn't handle a click on the graph because in the free edition it would open their site) and showing a link in the bottom of the chart. It seems it follows the same rule now according to their download page, so consider it if you want a good free charting library (I used also amMap and it was ok)
I implemented amCharts earlier this year on an internal project. The version we used is not rendered in flash. They use SVG (and VML for IE6-8 support). Been very pleased with their look/feel and feature set. The documentation on their site was organized and helpful. Concluded on amCharts as a result of a personal analysis of eight charting libraries. amCharts does allow free (with conditions, link) and pay options.
I think highcharts is a top contender but ultimately felt amCharts had better aesthetics. The updated highcharts is certainly looking good.
P.S. at that time (4-5 years ago) it was possible to use Amcharts for free in commercial app but with reduced features (for example you couldn't handle a click on the graph because in the free edition it would open their site) and showing a link in the bottom of the chart. It seems it follows the same rule now according to their download page, so consider it if you want a good free charting library (I used also amMap and it was ok)