I spent a fair while futzing about with ajax.net before deciding that it was either too brittle (particularly with the jscript in IE) or I was simply too stupid (I'm leaning strongly towards that one) for me to use it in my production sites, and started
hunting about for alternate "wrapper" solutions.
I settled on Telerik’s ajax solutions, which they’ve recently been updating to their “Prometheus” release (son of Ajax, or something like that) and it just makes my life so much easier. Pretty much “Ajax
for Dummies” which suits me just fine... and I can get stuff to work X-Browser/X-Platform with relative ease... albeit at a $cost and some acceptable (to me) performance/functionality trade-offs.