John Gleason's posting of Russ Croman's image of M42 reminded me of a very good paper written by Bessell et al entitled "Beauty and Astrophysics". In the paper he speaks to the issue of using emission line filters for astronomical images and the scientific value of such use.
He also has a number of example images showing such techniques.
here is a link to the paper.
http://www.atnf.csiro.au/pasa/17_2/bessell/paper/
Note that you can download the entire paper in PDF here:
http://www.atnf.csiro.au/pasa/17_2/bessell/paper.pdf
the images referenced in the paper can be found here:
http://www.mso.anu.edu.au/~bessell/thumbnails/
Although a scientific paper, it is an easy read.
To me, narrowband imaging is where the action is.
Richard