I have to say that I wasn't too comfortable with CSS before I bought this extension, so I kept designing sites with tables for structure for quite a while. You really need to sit and play with CSS sculptor for a good few hours to figure out what it is capable of.
Start with a preset; I often use a fixed width 2 or 3 column with a header and footer and often include navigation. From there I play with the settings, click apply and then click the preview to see how it sits in a browser. This tool has been invaluable for my education in CSS and I am now far more comfortable than I ever thought I would be. The big benefit of CSS is that if you make a design change, you just upload the CSS file... if you make a change to a table, it will more than likely be within a template, and then you have to upload every page that is based on that template.
There was one occasion where CSS proved to be more effort and way more time than using a table, so, I used a table, but it was a simple 4 cell table. A header, 2 columns, and a footer. Each cell was separated by 20 pixels so I could display the background between each cell, but I wanted the two columns to be the same height... it turned out to be far easier and more stable across all browsers to use a table. However, if I need to make a change to the design as a whole, I have to change the template and upload every page on my site.