I don’t think that improperly used tables have much effect on the users browsing your actual site, but using tables will make your site much harder to maintain, and alter which in the end will cost the client more money.
Considering that you will pay between $50 and $90 an hour for a good theme designer that added time becomes pricey very quickly. It takes so much more time in fact, that I can rebuild an entirely new theme faster than I can remodel a table based site. (Which is why I do just that when confronted with a tabled redesign or repair.)
CSS is also capable of delivering much better looking sites than table based sites.
Take a look at www.csszengarden.com for some examples. You won’t find the caliber of layout shown on csszengarden ever done with tables.
Once you master CSS you will wonder why in the hell you ever touched a tabled layout. CSS makes site alterations soooo much easier. I know because I used to use tables when I was getting started years ago.