Thinking as a user now, when do you find Ajax positivity impacts your experience, and when does it negatively so?
The main advantage of ajax not need to refresh the page to get the content dynamically, but today mostly use jquery a light weight ajax library that is used , main advantage is fast processing.
imho, on a page based on dynamically loaded subpages, it can be annoying when no location.hash-function is implemented, so you can't directly link to a specific page or content on a website. Doesn't really matter on small ones, but bigger pages or blogs that have ajax-functionality like this implemented, it's really important
Ajax can be frustrating when the website doesn't account for people not using it. Although there aren't many people who still block javascript, graceful degradation is important.
Guess that's the only drawback of this technology. That was the reason why I stopped using it in my portfolio. But generally, I'm fond of ajax. Given that it's so easily implemented through jQuery, for example, I believe it to be very useful.
As well as pop-ups, slow loading because they prefer "fancy" presentation like sliding a new screens across the screen IN SLOW MOTION instead of just displaying the content.
AJAX annoys me when i see a big thick book about it. There is not really much to it as far as howto. its not an entire programming language with classes, libraries. I hesitated to learn AJAX a long time ago due to the fact it seemed to complicated. I threw a script to handle GET ajax, and a script to handle POST ajax...and then i was done and ready to move on. Learning AJAX helped my process by a great percentage. it is a very valuable technique. Learning it and using it was anti-climactic considering the emphasis on Ajax given by web people.