In fact, its interesting that their new engine is so fast. Its going to get very interesting, especially since personally, I couldn't give a damn about supporting unfinalised standards the ACID3 tests (in fact, one of the ACID3 tests can't be passed 100% of the time anyway).
Speed is important, and since a lot of applications use the IE engine, maybe this will speed up some other applications as well.
Btw, I can confirm their sunspider benchmark.. Their preview does run faster than Firefox on my system (although, unsure if my addons are having an impact on that, or external causes). As its just a preview, it should only be taken with a grain of salt anyway..