For last couple of weeks i got quite a lot queries from team on Ajax. Its more of a standard response from me "use it, if it makes sense!".
IMHO one has to weigh the Ajax features against the coding complexity. Enter enterprise apps scenario, you got lot of additional things to consider, most importantly code maintainability and alternate technology options. Comparing enterprise apps against google map, google spreadsheet, flickr is like comparing apples and oranges. I'd seriously consider SmartClient if we really needed a windows feel in web.
Bottom line, page postback is not always evil !