Just my $0.02 - I'm in the middle of a protracted FreeNAS installation (my first). I've also FWIW used pfSense for about 5 years, since long before 2.3 and the new GUI there, and develop on FreeBSD. As a complete outsider to FreeNAS and hopeful long-term future user, I just tried 10 Beta 2, and loved the job you've done on the GUI.
While the modern UI is a matter of taste (I like it, some won't, that's fine), it is definitely much simpler for users, and much more straightforward to navigate. I found myself hoping this wasn't achieved at the cost of removing the more advanced options available in the 9.10 GUI, which I expect I'll need at some point., but if the aim was to make it easier, more intuitive and therefore also simpler to support, the FreeNAS 10 team has definitely from my POV succeeded. So much so I'm almost tempted to wait until March and install my new server directly onto 10 rather than on 9.10 and upgrade.
To my mind the biggest part of that success isn't so much to polish and smoothness (which I can live without), it's that unlike 9.10 and earlier, the layers of multiple access points where some options and functions were in the left sidebar, others in the top icon bar, and I had to hunt for things, or wonder if I'd missed them, is now much more straightforward - it's been nicely centralised in one bar on the left, and much more intuitive to navigate when new to it, and to find what one's after. With luck, theming and css file mods can address some of the layout changes for people who prefer the older layout or want denser information. The only negative is that it felt slightly slower to load and respond to actions in the browser, but it's still a beta so we'll see. Not a big deal anyway.
I hope that this isn't at the cost of loss of capability and power, but I'm sure it isn't. I look forward to beta 3 and the final release - and hopefully this is useful feedback for the team that its work is very appreciated, even if some may prefer the UI that was used previously.