An extension to the StringBuilder
A couple of quick comments:

You can use the see tag's cref attribute. If you generate documentation, some tools will generate hyperlinks for you.

/// &lt;summary&gt;
/// This wraps the .NET &lt;see cref="StringBuilder"/&gt; in a slightly easier to use format.
/// &lt;/summary&gt;




The length property of a StringBuilder is read and writable. It's also really useful for it to be so:

var sb = new StringBuilder();
foreach (var i in Enumerable.Range(0,10))
{
    sb.AppendFormat("{0},", i);
}
sb.Length--; // removes the trailing comma.


That's a contrived example which is trivially served with string.Join but setting the length can be useful!



The _stringBuilder field should be readonly.



I'd say CurrentString is superflous. Just call _stringBuilder.ToString()



I must admit, personally I think the StringBuilder api is really good, I've never needed an implicit conversion to a string or felt the need to + rather than append to them.