I've got an external overflow, and like it. The only real difference between mine and the one on the link, is ine is much shorter... 6"deep I think.
One thing I wish I had done, is to make the overflow longr. Make it go from end of tank to end of tank, It works out to be a great little place to grow frags, and macro. Basically like a little refugium.
Just make your drain pipe extend a few inches up into the overflow, so there is always a few inches of water in there.
Chris