\begin{frame}[c]{Testimonials}
  \begin{onlyenv}<1>
    Martin Fowler, 2010:\hfill\curl{http://martinfowler.com/}

    \begin{center}
      \includegraphics[width=0.75\textwidth]{vcs-plane}
    \end{center}
  \end{onlyenv}

  \begin{onlyenv}<2>
    Joel Spolsky, 2010:\hfill\curl{http://www.joelonsoftware.com/}\\
    \vspace{-\bigskipamount}
    \begin{center}
    \begin{tikzpicture}
      \node[draw=structure, fill=structure!20!white, text justified,
        text width=0.95\textwidth, font=\small]{
    Mercurial is better than Subversion.

    It is a better way of working on source code with a team. It is
    a better way of working on source code by yourself.

    It is just better.};
    \end{tikzpicture}
    \end{center}
  \end{onlyenv}
\end{frame}