Do whatever is easiest first. JS can be pretty, but hard -- at least, the DOM is much harder than HTML. Get an HTML serialization of your data working first, before you try to use client-side JS to write this data dynamically into a form -- not because it helps us, but because JS mappings are harder than PHP echos.
It will also give you a better appreciation of the places where Web 2.0 can really streamline a system. Just to give an example, Gmail is a massive JS app which uses a frankly unbelievable number of divs to reimplement an iframe window where you can view your email and/or lists of email subjects. It's quite possible that the communications reduction is so big that this is important to Gmail, but you're not that size yet, so just use an iframe rather than reimplementing that functionality in a special way.
There are other situations where JS is a bad technology. I should be able to navigate your site without JS, and if you demand JS for navigation you're probably doing it wrong. I should potentially even be able to log in, if you're not using OpenID or BrowserID or client-side crypto.
AJAX can be useful for creating chat applications, or for situations where you want to be able to see, query, and throw away lots of little pieces of information. Javascript is also useful when you want a control which should never hit the server, like folding a tree of comments -- which I recently implemented as a user script for HN.
That's another plus of using HTML, by the way: it makes it easier for scripters to hack on your site to add their own personal features. I tried to do user scripting on Gmail at one point, it was damn near impossible. Their divs belong to memorably named classes like "vI8oZc cN" and "nH w-asV" and "mq nH oy8Mbf". Such are the perils of trying to build your iframes dynamically out of divs.
Anyway, once you start to get into user interactions, JS becomes much more fun and important. If I am coming to your site to play an HTML5 game, then I already know I need to turn off NoScript, you don't have to tell me. If you've got an interaction which simply screams "drag and drop", then do that instead. Some of the nice uses of JS I've seen recently amount to visualizing graph networks and allow you to drag nodes around to optimize the display; that's a good candidate for a JS implementation.
JS is not merely for facilitation, and can have real uses on a CRUD-type site. Just be sure that you're not reinventing something which already exists without JS, like iframes, URLs, and so forth.