In theory it has the best AJAX support of any browser - in practice it proved why AJAX is not fit for mobile UIs yet. I ran a quick test on Flickr, the kind of site that ought to be in the standard test suite for web 2.0 browsers I'd have thought, and tried to tag a photo (which uses some funky AJAX, for those that don't have Flickr accounts).
It took me about 2 minutes to navigate through to a photo - various clicks did not do what I expected (ie. what they do on a desktop) - during which time I decided that the zooming system is quite clever, but ultimately quite annoying and not as good as a nicely reformatted page which takes account of the screen size (like Webkit / Opera) or indeed anywhere near as nice as a pinch and zoom iPhone affair.
Despite the browser emulating a mouse pointer (I always dislike this as a UI system with handsets that don't have proper pointing hardware), CSS rollovers weren't shown at all which is a shame as the new Flickr home page makes very good use of them when showing you recent events.
On reaching the photo I wanted to tag, I clicked on the 'Add a tag' link and waited. And waited. (A simultaneous test on my laptop on another photo suggested it was not Flickr slowing things down). After 4-5 seconds a textbox appeared, I entered the tag, clicked 'Add' and waited several more seconds. Then the tag appeared.
So on the one hand, Skyfire handled the AJAX as well as it could within the constraints of the handset. But on the other, whilst faster than reloading whole pages, the AJAX experience wasn't exactly super quick and responsive - something I find essential for me to bother using AJAXy features on a real PC.
The AJAX speed isn't Skyfire's fault of course, it just shows the limitations of the medium - many years after 3G was launched it still isn't widespread and still isn't fast. Maybe HSDPA will work better, but the whole experience didn't convince me that I want to do anything serious in a mobile browser any year soon...