I've had it installed for some time, mainly for testing sites with.

Yes it is much faster than previous versions. It is on par with Chrome. It is also much faster than Chrome and the others in some areas as far more of what it does uses the GPU to accelerate stuff.

I'm still using Chrome as my desktop browser though, it still feels cleaner than IE9 and has a bunch of extensions that I wouldn't want to start looking for IE equivalents for.

The biggest win for us developers though is that its rendering is better, meaning once more end users are using it we'll have less layout hassle. Or at least we would if we could convince our corporate clients not to support IE6 and IE7