It's pretty good, albeit much less good since the old XPCOM extension support went away. (My position is that XPCOM compatibility could have been maintained in a multi-process world through better dynamic object proxying.) But as Mozilla's money dries up, so will Gecko, Servo, and a bunch of other fascinating projects. It's sad. But the web won't stop evolving, so we're going to end up a webkit-only world.