In general I have to say that from what i have seen is that when it works it's great. when it fails it sux big time.
i have seen problems on not just the MSFT versions but others also.
there is a web mail system i use that very often does an html "pop up" that just hangs.... i have to close and re-open the browser.
i have not tested with different browsers so it's possible the common thread is IE ?
so ither IE does a poor job with ajax or there are fundamental problems with realizing the concept of ajax today.
my future outlook is that web sites i will build will only use ajax in very carefully selected places.
for more involved UI and page logic i will see if silverlight is a "better ajax".