There's no denying the usefulness of narrowband and line filters for the observation of emission and planetary nebulae and supernova remnants. In some cases, nebulae that are invisible without a filter can be seen. In others, marginal objects can be enhanced markedly.
Well-known DSOs like NGC 7000 (The North America Nebula), the Veil Nebula complex, NGC 2359 (Thor's Helmet), and NGC 7293 (the Helix Nebula) are all vastly improved with the use of a narrowband or OIII filter.
Many tiny planetary nebulae may not detectable unless they are first "blinked" with a nebula filter.
Browse http://www.astronomy-mall.com/Adventures.In.Deep.Space/barnard.htm for a discussion of observing the Hubble HII regions in NGC 6822.
Some years ago the noted amateur astronomer David Knisely conducted a systematic evaluation of the various type of filters on a large number of objects. It can be found at http://www.prairieastronomyclub.org/resources/by-dave-knisely/filter-performance-comparisons-for-some-common-nebulae/
While I've successfully observed B33 (the Horsehead Nebula) without an H-beta line filter on a few occasions when the conditions at some excellent dark sites were truly exceptional and large apertures (14.5 to 20 inches) were employed, the addition of an H-beta filter can make logging the Horsehead relatively "easy". An H-beta filter also makes NGC 1499 (the California Nebula) a worthwhile target, especially with a rich-field telescope like my Tele Vue TV-101 apochromatic refractor.
Here's David Knisely's list of observable H-beta emitting nebulae, many of which are otherwise invisible without an H-beta filter:
1. IC 434 (HORSEHEAD NEBULA)
2. NGC 1499 (CALIFORNIA NEBULA, naked eye and RFT)
3. M43 (part of the Great Orion Nebula)
4. IC 5146 (COCOON NEBULA in Cygnus)
5. M20 (TRIFID NEBULA, main section)
6. NGC 2327 (diffuse nebula in Monoceros)
7. IC 405 (the FLAMING STAR NEBULA in Auriga)
8. IC 417 (diffuse Nebula in Auriga)
9. IC 1283 (diffuse Nebula in Sagittarius)
10. IC 1318 GAMMA CYGNI NEBULA (diffuse nebula in Cygnus)
11. IC 2177: (Diffuse Nebula, Monoceros)
12. IC 5076 (diffuse nebula, Cygnus)
13. PK64+5.1 "CAMPBELL'S HYDROGEN STAR" Cygnus (PNG 64.7+5.0)
14. Sh2-157a (small round nebula inside larger Sh2-157, Cassiopeia)
15. Sh2-235 (diffuse nebula in Auriga).
16. Sh2-276 "BARNARD'S LOOP" (diffuse nebula in Orion, naked eye)
17. IC 2162 (diffuse nebula in northern Orion)
18 Sh2-254 (diffuse nebula in northern Orion near IC 2162)
19. Sh2-256-7 (diffuse nebula in northern Orion near IC 2162)
20. vdB93 (Gum-1) (diffuse nebula in Monoceros near IC 2177)
21. Lambda Orionis nebular complex (very large, naked-eye)
22. The "Cone" Nebula (portion of Sh2-273 south of cluster NGC 2264)
The GCE filter, which supposedly enhanced galaxies, was marketed some time ago.