Update: I got a lot of comments on my post. I am trying to answer them as they come in, so check back after you leave a comment.
We used Apple’s SunSpider benchmarks for our tests. Depending on the OS and machine configuration we are 1.18x to 1.28x faster than V8. Since V8 is only available for Windows, we didn’t perform any tests on MacOSX and Linux, both of which we support already. Our latest builds also work on ARM, by the way.
Talking about IE, our tests also indicate that we are about 15 times faster than IE 7, and about 5 times faster than IE 8 beta on the SunSpider aggregate scores.
If you want to give TraceMonkey a try, take a look at our nightly builds. You can enable the JIT in the about:config settings. The nightly builds are certainly not ready yet for wide-spread use, but we have improved stability significantly since our initial preview release. Firefox with TraceMonkey enabled is now my default browser, and I am writing this post with it.