I converted my program from Delphi 4 to Delphi 2009 a year ago, mainly to make the jump to Unicode, but also to gain the benefits of all those years of Delphi improvements.<
TStringBuilder is basically just a me-too feature, like LachlanG said. It's needed in .NET because CLR strings are immutable, but Delphi doesn't have that problem so it doesn't really require a string builder as a workaround.