<p>Are <a href="http://jquery.com/demo/thickbox/" rel="noreferrer">modal windows/dialogs</a> considered a good practice for websites?  One of the things that concerns me about using a modal window is that it is so much like a pop-up window and it could cause user frustration or cause users to simply close it because it's intrusive. Do you know of any studies that talk about user preferences and use rates? </p>

<p>Are there any accessibility issues beyond having the a modal window link load the content in a new page when JavaScript is turned off? Do you <em>like</em> sites or web apps that use modal windows? Why or why not? What are good and bad practices for implementing modal windows? I know modals are probably most often used when displaying pictures or galleries, but what are some other practical use cases? Do you use them on your site and what lead you to your decision? </p>

From a practice standpoint - excluding accessibility - modal windows provide an alternative that are less startling as, say, dialog boxes and feel less intrusive than pop-up windows. They usually have a more cohesive feel to the site than either of the two aforementioned method. Aside from pictures or media, I've used modal dialogs to serve the same purpose as dialog boxes - to get the user to give some form of required input, or to acknowledge something before letting them interact with the site again.
From a purely accessible standpoint, they aren't so great. They typically require JavaScript, and, because of the way that modals are managed with respect to the DOM, screen readers will not interpret them well. To combat this, it's always a good idea to degrade gracefully - one suggestion is to take the content of the modal dialog and place it on a page. Whenever the modal dialog
should
appear, have the current page redirect to the 'modal page', get the user's input, and return to the page it was on.
Finally, as far as my personal opinions are concerned, I don't mind modal windows. I think that, when used correctly, they provide a decent experience - if something
has
to get my attention, then I'd rather it do so without redirecting me from the page, showing an ugly operating system themed dialog, or a pop-up (which my browser will likely block, anyway). I guess, to sum it up, if I had to choose
how
I'd like to be interrupted, then I'd choose a modal dialog.