Underlining and color are two clues for your visitor of a hyperlink. If you take away the clues, the site may look better but the novice is confused and clicks away in frustration. The advantage of buttons is that when used with ALT tags, a site can be navigated without images on. The disadvantages are that each button graphic requires a separate call to the server, so your visitor has to wait for each to download before he can see your navigation system. Image Maps are graphical menus which have a map linked to the position you put your cursor.
Image maps look good, and download faster than buttons that cover the same screen area.
But neither browsers with images off nor search engines can follow them. They're also more difficult to maintain than either text or button menus. Another way to get your customers deep into your site quickly is to place a drop-down menu on your main page, with sections clarified by indentations or spaces. Such menus don't take up much screen real estate at all, but pack a lot of navigation power. The downside is that your customer must click on the dropdown menu in order to see the contents.