In general, I recommend avoiding any dependency on JavaScript in a Web site. A high enough proportion of Web users have JavaScript disabled to cause problems. Popular sites relying on JavaScript that I've worked on tend to receive a small amount of mail from users who have problems using the site. I suspect most users encountering problems go elsewhere without contacting anyone.

Most features that JavaScript offers can be recreated using a combination of HTML, server side code and other techniques that will function in a wide range of environments. Several features of JavaScript (pop-up windows, window resizing) are annoying, so they're worth avoiding anyway.

One advantage of client-side code is that no round-trip to the server is required. I recommend using JavaScript form validation when your budget allows. Of course, server side validation is essential too - never trust client software!

JavaScript can add to the user's experience in some ways, such as automatic redirection to another page within SELECT lists. I recommend using JavaScript in such cases, provided you think carefully about how the site will work when JavaScript is disabled or unavailable.

In short, I see no harm in using JavaScript to enhance your site, provided it is used only for enhancement and not as an essential part of the experience.