I loves it. I loves it like I loves Soda-Pop, and that's a lot. It runs much faster, looks amazing (at least in Ubuntu and Windows), and has so many nifty features!
Application tabs are awesome. Very awesome. I was afraid they would slow startup, but it doesn't seem to be much of an issue, and I've got a few. They're there for those "web-apps" that you leave open pretty much all the time, and they stay there until you tell Firefox otherwise. There are a few issues I have with that feature, though. If you navigate away from the "app" address in the same tab it changes the URL associated with that tab until you change it back. I also feel that they could stand to be paused or have to be initiated, rather than live, in the event that resources become an issue. Easy enough to solve both of those in the future: app tabs could be locked to the domain; they don't need to load until you've shown that you're actually going to use it in this session, and they can idle out after a set time of inactivity.
Sync! I already used it. Extremely handy for me, considering I run at least a dual boot on all my machines, and and triple on my primary. It's as good as ever, and you don't realise you need it until you have it.
Tab groups, I don't use. Yet.
The interface is so clean and integrates with the OS so well. Of course it also frees up more viewable area. One might say they copied off of Chrome, but bear in mind all of what Firefox has pioneered in web browsers that has since become the industry standard. Also, Chrome has basically zero OS integration, which can completely ruin the unified look across a desktop.
I was going to wait until they pushed it as an update, but I'm glad I didn't. I advise anyone who has used Firefox in the past give it a look-see.