[jblock]

For one, it's heavy compared to some very simple JS and a good portion of the time you don't need the entirety of the library.

That said, it does abstract much of the crap that goes along with XMLHttpRequests and all the cross-browser idiosyncrasies that go along with it.

I'd say at this point it's a matter of personal preference and performance requirements.

[drgath]

I think a combination of something like TinyXHR (https://gist.github.com/shimondoodkin/4706967) and querySelectorAll would suffice for many simple use-cases on modern browsers.