frames & framesets are a very usefull tool, however they are not really liked by the web hierarchy & academics. They are also rarely well handled by robots & spiders. As a consequence they are deprecated.
you can achieve most of what you can in framesets by using CSS, however I've never yet found the CSS solution to the idea of the outer frame(s) remaining in view whilst the visitor reads the main page contnet.. meaning that the site nav tends to dissapear 'off screen'. They are a hige bonus if you design using a free flowing text paradigm.. allowing the users to make the text as big (or small) as they feel comfortable with.
I think any web developer these days ought to be using or at least seriously considering using CSS. Advocates claim they are easy to use, and logical. I find them a pain, difficult to debug, irrational and overly fiddly.
I recently had to get a fairly complex layout into a CSS... the base PHP coding & a PHP layout using absolute sizes took about a day and half. pushing the same basic code into CSS took weeks..... Im not kidding. If you are used to CSS then it may be easier. but to me its a major stumbling block to rapid development. I can see the power of CSS if say your customer has a bad hair day an flounces in demanding changes to colours, fonts etc... adpating the CSS to meet their latest brainwave is definately easier than flashing up the code editor.
So if you have a CSS expert on your staff then CSS is a great way to go, otherwise be prepared for some big struggles. Its alleged that the initial pain of learning CSS pays off huge dividends in the long term. to me its a neccesary evil.
if you do go down the CSS route consider getting a competant CSS editor.. something like TopStyle may be a good investment.
I'd rather be riding on the Tiger 800 or the Norton