XmlHttpRequest – Which one to pick?

After having GMail, Google Suggests, Google Maps, Yahoo! mail contact list, it seems like XmlHttpRequest is really kicking in.

Althought the idea have been around for a while yet, it looks like Google Maps smooth zooming and panning have hit people on the heads (I love it).

I’m reading about Ajax, DWR, SWF and it seems that the craze is starting again, which one to pick, which one to invest in.