With the addtion of Extension methods to C# we've seen a lot of them crop up in our group. One debate revolves around extension methods like this one:

public static class StringExt
{
    /// <summary>
    /// Shortcut for string.Format.
    /// </summary>
    /// <param name="str"></param>
    /// <param name="args"></param>
    /// <returns></returns>
    public static string Format(this string str, params object[] args)
    {
        if (str == null) return null;
        return string.Format(str, args);
    }
}

Does this extension method break any programming best practices that you can name? Would you use it anyway, if not why? If I renamed the function to "F" but left the xml comments would that be epic fail or just a wonderful savings of keystrokes?