I tried Hummingbird out this afternoon.
Before activating I tested my site speed at GTMetrix.com and got a B rating and 89% score on Page Insights.
I have been using WP FastestCache, so I disabled that and re-tested the site. The result dropped to 53% and an E rating (so FC was doing its job).
I then activated Hummingbird, set all of the parameters and tested again with GT Metrix. The result returned to 89% and a B rating, so that's working as it should too.
However, when I monitor uptime, Hummingbird is reporting that my site is not publicly available. I know it is, so there's a false positive somewhere. Just letting you know in case it's a bug.