I've tried Orion a couple of times. I even used it as my default browser for ~3 months about two years ago. Most recently, I tried to use it again about 2 months ago, but it still had loads of bugs and, most of all, was painfully slow.
The truth is, Orion being based off of WebKit comes with the obvious limitation that....it's based off of WebKit! So much slower than chrome or firefox, and plagued with decisions that are just not to my taste. For example, just the way it behaves when I hit the back button (or, rather, when I swipe back) feels incredibly sluggish. Loading is often terrible, with constant repaints of the screen as well. A bunch of websites don't work properly either.
The only true reason why I wanted Orion to work was because I wanted a browser that would be good for my battery life and "optimized for the mac". But, since then, I've realized I don't really use the battery that much (or that I don't notice it being a problem), and that, whatever "optimized for the mac" means, it definitely isn't speed.
After Arc went around and poo-pooed on its users, I migrated to Zen (I did try Orion again, like I mentioned). Zen is also filled with bugs, but at least I don't want to throw my computer out the window because of it being slow.