Checking it out now - performance seems pretty snappy to me, and the Firefox import worked flawlessly
How it renders pages varies wildly, however. Some of my other forums render normally (same as in Firefox), some are slightly different - and a couple are just plain awful
Unfortunately, one of the awful ones is Ebay - fonts so tiny that they're nearly unreadable, and I haven't found any tweak yet that will fix it. (the tools menu is pretty limited so far).
Even today, that's a pretty fatal one to get wrong - so I don't think they were really ready for beta mode yet. I'll check back in a few weeks; I like the concept, but it looks like they jumped the gun a bit releasing it.