My experience to date is that they do different things on different objects under different sky conditions (light pollution mainly).
My current set (all 2") is:
DGM Optics NPB filter
DGM Optics OIII filter
Baader UHC-S filter
The DGM filters are sold by Omega Astrofilters. There is good detail on each filter on their site, including spectral transmission curves:-
http://www.omegafiltersebuyer.com/servlet/StoreFront
I recommend that you look for filter reviews by David Knisely. I think you'll find them on the Cloudy Nights forum.
Edit:- Here are some of his reviews:-
http://www.prairieastronomyclub.org/filters.htm
http://pages.sbcglobal.net/raycash/filters.htm
http://www.cloudynights.com/byauthor.php?author_id=2 (look on page 2 for reviews of the DGM Optics filters)
I love my NPB filter. It is magic on the Carina nebula.
I have used another brand of OIII filter for successfully increasing the contrast on planetary nebula, very useful when they are small and hard to distinguish from the starfield. Hopefuly the DGM OIII will do the same job.
I have looked at NGC 253 in light polluted skies using the UHC-S filter and thought - that's not much better than without the filter?
So I have plenty of testing to do. When I have a filter slide, I will mount all three and will be able to quickly swap between them for some rigorous testing.