I've been using css for years to style my pages, but until the last few months had stuck with tables for layout. The reason for the change was I was developing a site for a public body, and accessibility was a big factor.
Now that I have made the change I would *never* go back.
Obviously there are various inconsistencies with the browsers, however, once you get to know them they are easily worked round. You can either use the hacks, or just work your designs in a way where they look ok with either display. I'm basically designing for Firefox and testing compliance in IE7 and IE6. Hopefully over the next couple of years as standards are embraced the quirks will become less and less.
I recently converted our
Girls Night site into a pure css layout and reduced the code from around 15k in tables, to 4k in css. The page is around 8k, with 4k of code and 3 and a bit k of text. Obviously this is going to result in an immediate speed up of load time, however, the other advantage of css over tables is that elements load independently of each other, whereas a table will only load once all the content within has loaded.
The navigation bar I converted from complex roll over graphics, to a simple ordered list of links - accessible and lite on code.
As I said, once you get into the way of thinking, you'll never want to go back to tables.