If you were to do a search on the forums here on Omega Optics, or possibly DGM NPB filter you'd find that there are plenty of us here who own and love these useful and affordable filters.
The NPB filter (stands for Narrow Pass band) is basically a UHC filter (I've been told that UHC is a proprietory name so technically only the original company can use that name).
By it's very specific design it cuts out certain groups of specific (less desireable) light frequencies, leaving a beter ratio of "the good stuff" that allows us to see many nebulae with more contrast.
An OIII (Oxygen-3) filter works similarly, but cutting and allowing different light frequencies. Some DSO's respond better to an OIII filter, but generally more are improved by the UHC type. These filters do not "increase" the amount of light you see in the desired frequencies, they block unwanted freq's thus giving you a better ratio of good to bad freq's, therefore improving the contrast of the object you are trying to view.
The Carina Nebula for instance when viewed through an NPB filter is a majestic site to behold - ask anyone who was within earshot the first time I tried mine out!
Lets see if I can get some more info here for you....
Their storefront page:
The Good Guts on the NPB filter, including pricing and options for "cosmetic seconds" at approx 25% off standard price.
I purchased one of the 1.25" variety a year or more ago, now I thought that near $30 bucks aussie was a bit strong to send somethign that small, but I was amazed to say the least to have it turn up by courier at my door only 3 days after I placed the order online!!! This is after having flown form the US, I've had letters take more time that that to travel 80 km's to my place from Brisbane.
The NPB won't do squat for planets or galaxies though, and I can't comment on the effectivenes of the DGM GCE "Galaxy contrast" filter. Perhaps someone else here has tried one.
Some simple colour filters can help with planetary details. I have used a pale blue (80a or 82a?) filter to bring out the banding on Jupiter for instance. (available at several vendors like Andrews or Bintel for around $10-12 bucks per filter I think, very inexpensive).
You will always find adds around for filters that remove street light glare, etc - some work better than others apparently but at the end fo the day (ie at Night!) there is no filter substitute in that regard for a good dark sky.