Hi folks... let me see if I can address some of the questions/issues raised in the first few posts...
About it being broken in various browsers... yes, that's true to a degree. However, over the past maybe 2-3 years, the brwosers have all pretty well implemented ECMA standards, so you won't find a whole lot of issues these days. True enough, some still exist, and of course they tend to bite you hardest on a weekend at 10:30 at night trying to meet a Monday deadline :) I for one don't think calling the situation "horrible" is really fair any more... a few years ago, absolutely yes, I'd agree, but not today. Today its managable, albeit not perfect, if you stick to standards.
As Sergey mentions, most of the time, aside from debugging, you don't have to know all the details of HTTP to do web development. Likewise, having to know all the details of XMLHttpRequest is generally not required. There will always be times when that knowledge helps, but generally, it's extra.
As I say pretty clearly in my book, AJAX is far from a new concept. I have an app, still in production today, that I wrote in 1998. You hit the initial URL, and you download about 300k of markup, most of it in hidden
s. Then, as you work, certain
, and shows the next one. This is AJAX without XML, or XMLHttpRequest. Of course, I didn't know I was doing anything special at the time or I would have given it a name and been all rich and famous now! ;) To me, it was just an approach that made sense.